your doing a very similar setup to me i think....ive got a 1981 301 (ecu equipped) TA th350 3.08 gears and am putting a 1970 400 in.
with decent heads with 9 or over cr and good engine and a decent cam will c u into the 300+
3.08 gears would be good...at least they are for me...i have no intention of changing mine anyway
its easy to get the motor out without any damage and i found it a reall easy job...took 4-5 hours to get radiator away and bits and get the motor and leave the tranny there....400 depending on year shoud have same mounts (post 1970 i think anyway) so should bolt right back in place of the 301.
intake will sit higher because 400 has higher block hight so sum adjustmant may be neccasary but not a big problem
standard and after market headers should fit fine
think a 4 core is a good inversment. dont cut corners on basics like that and stock fan should be fine for that hp range and i dont think you need a new fan shroud no.(check that thou)
now im no expert...all of this is down to personl experiance so i would double check in case sum of the experts here know differant
