Making torque at higher RPM's involves making sure all the parts that flow air are all up to the task, and somewhat in synch with each other. Torque at high RPM's involves moving more air through the motor, air + gas = HP.
The carbs are the first step in that flow (well, air cleaners, but don't get too hyped up on air cleaners as something that's holding your car back). I have no experience with Mikuni's, just with SU and Weber DCOE setups (along with D-Jet, but that's a different animal). The SU carbs were very competent, and could be modified to keep up with engine mods, for a fair bit. The basic size of the carb pair was a bit overspecced for Volvo's stock HP.
As the engine mods on my PV's motor accumulated, eventually I swapped to the DCOE's. Both made plenty of low end torque, but the DCOE's kept that power going higher in the RPM range. Plus, they're more forgiving of the erratic airflow produced by a lumpy high revving cam with lots of overlap at lower RPM's. The SU's are paying careful attention to the pressure in the venturi, and positioning the mixture piston accordingly, having pressure waves coming back up out of the intakes is not helpful. The DCOE's are more 'steady state', all they do is possibly re-gas some already gassed air if it goes out and back in again.
Next link in the chain is the head itself. The valves and the ports. B18's had smaller intake valves, B20's had larger, and the injected B20's even larger. But they all kept the same exhaust valve sizes. And the exhaust ports are very awkward in terms of flow. Because the intake and exhaust is on the same side of the engine, packaging demanded that the exhaust ports be lower in the head. So they make a very sharp turn in the bowl, more than 90 degrees, and proceed out only a quarter inch or so above the bottom of the head. This is not great for flow, and while it can be improved, it can also be damaged by someone porting out the bowls that doesn't pay attention to what they are doing. But in general, bigger valves, work done on the ports on the exhaust side, and double valve springs will all get the head to flow more air at higher RPM's. Double valve springs help the valves close at higher RPM's. And OHV engine has more hardware that needs to move around as the valves open and close, the springs need to pull the valve closed, plus the rocker arm, and the push rod, and the lifter. So stiffer is better, plus the double springs also don't have a static resonance, like a single spring (can).
Next is the cam. The shape of the lobes, and research over the decades makes newer shapes that work better over wider RPM ranges. Old style cams (and you'll find lots of these for sale for the OHV Volvo motors, because they're similarly old) will have much more pronounced RPM ranges they're effective at. In addition to the lobe shape, there's the timing of the valve openings, the amount of lift (how far the valve opens), and how long the exhaust and intake valves are open at the same time. It is important to note here that a cam is (to a degree) tuned for peak efficiency at a given RPM, and this is rather pointless if the rest of the air moving parts are not effective at a similar RPM range. It all needs to work together.
Next up is the exhaust manifold. There's more to an exhaust manifold than in merely directing exhaust away from the motor and out back somewhere where it won't asphyxiate you. Simply getting the last exhaust gas pulse out of the way isn't quite good enough here, what you want to do is get the last pulse to *aid* the next one. This is all based on the diameter of the exhaust runner, how long it is, and how long it takes for a pulse to reach a merged flow with the other cylinders. This all comes down to RPM tuning again, as the rpm's rise, the length of tubing needed to have the last pulse still heading away and producing a slight pull (relative term, to the ambient pressure in the exhaust system, whatever that is) at the exhaust valve when it opens. So the next pulse gets pulled into the exhaust, instead of needing to push its way out against pressure. This is often misunderstood as 'backpressure' - because pipes that are too large can hurt power at certain RPM's, because the exhaust is merely getting out of the way for the next pulse, instead of actively helping it along some. All a bunch of words to support the notion that the exhaust manifold is tuned for a certain RPM range as well. I'm certainly not a pro at this, but what I've read form people who are (somewhat?) is that the cheap 4:1 (all 4 tubes extend for a while and all merge into one) headers commonly for sale for old Volvo's are tuned for an RPM range so high as to not be commonly used in any sort of street driving situations. What does work better are the less common, usually a bit spendier 4:2:1 headers. The #1&4 cylinders pair up somewhat quickly, as do the #2&3 cylinders. This allows those pairs of cylinders to help pull exhaust from their twinned pair, effectively getting a pulse twice as often (every 360 degrees instead of every 720) and thus working at lower RPM's. Then those two joined tubes join together into 1 pipe further down. The stock exhaust system is already configured like that, and by all accounts works very well until you've got plenty of other mods piled up.
Past that, the exhaust system just needs to stay out of the way. Once all the cylinders are merged together, you just don't want the system to have any restrictions that prevent flow at high RPM/WOT.
I think an ignition system will only produce a 'pretty solid power boost' if the system you're comparing it to is pretty weak. A properly setup 70's era Volvo ignition system should have enough pep with the right plugs to not cause you any grief. I got some improvements on mine, but that was when I replaced a very old and very weak original early 1960's style coil. Later on, I put an MSD box on it as well, which makes multiple zaps at lower RPM's which seems to help starting, low end torque, but it's not a huge difference.