I've seen them cut at various places, I would see where you think it's easiest for you to do. I think 4" is plenty, there are a few companies that make recessed firewalls to replace yours if you were interested. *****in' is one ( I believe) but not the only one.
[ April 10, 2003: Message edited by: woodz428 ]</p>
1. Mount engine and tranny in chassis.
2. Set body on chassis behind and up close to engine.
3. Draw outline of engine on firewall. May need to do this several times cutting and fitting until you have the right clearance around engine and tranny. Make modest cuts and be patient.
4. Slide body back and cut and remove firewall @ engine outline.
5. Slide body back to it's final position on chassis and put mounting bolts in to hold position. Concurrently cut floor board to clear back of engine and tranny.
6. Fabricate firewall and tranny cover panels in place to get perfect clearance. Weld and bolt them accordingly.
The beauty of a MOdel A is that the firewall is removeable. Take it out and do your engine install with the body on the frame where it belongs. On an A you can practically build the whole car without the firewall installed. If you're not using the original cowl gas tank you may able to reinstall the firewall backwards and pick up the room you need. Of course you'll have a bunch of holes to fill and I'm sure trimming on the bottom but hey it's just work...