define 'semi reliable'.
And what you will need is:
- money
- time
- skills
The three interchange fairly well. Money allows you to buy other people's time and skills. Time allows you to fumble your way through stuff you don't know about. Skills allows you to do it right the first time.
Since you seem to have a firm HP level there (likely pulled from thin air) and you haven't done anything yet, you can use this point in the project to decide where you want to go.
- keep the 8V motor. It can make 400 hp. But it has to be pushed hard to do so. Which means it has to be done right. Which is not cheap, and the path for tuning that will keep the motor intact is getting fairly narrow. But it's cheap and easy to start off on the path and slowly work your way up toward 250 - 275 - 325-etc hp.
- 16 swap - Far easier to make 400 hp with a 16V head, since it just flows air so much better. But it's a somewhat pricey endeavor to get one with a turbo up and running in a 240, since nothing really fits. Custom intake and exhaust of some sort. More potential to make HP with this swap, but there's a somewhat pricey buy-in upfront.
- complete engine swap. Two of the more popular swaps are the Gen3/4 Chevy LS family of motors, the Toyota 2JZ. Both of those make yuuuuge HP's pretty easily and are very well supported by a performance aftermarket. Or any of a thousand other motors. There is a whole lot of work involved, if you're not doing it yourself, it's going to cost a silly amount of money. If you don't know what you're doing, hope you're a good learner, there's lots of info on the interwebs.
An LS can even make 400 hp without a turbo, and the aluminum blocked versions aren't even heavier than the iron block Volvo motor by much. And an N/A build sure makes things easier.