[dsraven]

with a 6 banger you may be able to pull it off if the plan is a firewall mounted brake booster and pedal set up and also a different than stock steering gear set up. otherwise those items may just be right in the middle of your free foot space anyway.
before you start I would say brace the cab, with the doors hung and adjusted fairly close to good, so everything stays in the shape it should be. do bracing in an X pattern, side to side across the dash area and fore to aft in the door openings. after that the doors could be removed for ease of access. keep in mind you will need to try to leave yourself some room to work with the bracing installed. when I braced my 57 cab I used some 3/4 black pipe because it is cheap. I installed a horizontal cross bar from side to side just high enough off the floor so a 4x4 would fit under it through the door openings. that made it easy to lift the cab by myself with a couple of long 4x4's and some saw horses. then used some X pattern braces from above there to each upper and lower corner. that held things solid. remember though, that you will want to check the seating position with your new firewall so try to leave foot room access if possible. then cut away the firewall only. mock up what you want to do with some heavy cardboard and duct tape and see if you like it and if it will fit around the engine and trans and still leave you foot room around the pedals. try to keep the seats and that part of the floor intact so you can sit in the cab with the firewall cut out. if it works then it's great. if it doesn't work then you planned on doing the firewall anyway. the floor wasn't cut out yet so that part wasn't affected.