Consider another trunk solution......Google "start 'em up" trunk battery kit. I used his complete setup in my 70 Malibu trunk battery conversion. Part of the genius is using a FORD starter solenoid in the trunk, so that the positive batt. cable is DEAD at all times unless you are cranking. The next move is to use the BMW battery box, or any NHRA sanctioned "vented box" so you don't build hydrogen fumes from your charging battery. If you want to see my Malibu setup, PM me and I'll link you to the photoalbum.
Company is MAD Electrical.....has made these trunk start kits for decades for musclecars, all forms of racing machines. If you call him, be prepared to listen for 1+ hour......