We are in the same boat. I have been saving a '68 convertible for 40 years and I finally have time to get at it. First off, without the factory 400 engine originality is down the drain. You may find a good 400 but it won't be original. But all the parts interchange easily so not a problem there. So prime the oiling system, see what kind of oil pressure you get. Then do a compression test on that 350. If good, you are golden. Then do a basic tune-up and see if it runs. Even 350s are getting scarce. 400s are great but 350s are just as good and will take all the same improvements. The average person will NOT be able to tell if it is a 350 or 400. Is it a 4 barrel or 2? 'Most' 350s are 2 barrel. Someone may have swapped over the 400 top end/intake, etc. A 400 car usually had a Turbo 400 auto trans. If that is what you have you are now double golden! Bulletproof is the word usually used to describe that tranny. I agree, a manual trans is better and all the stuff needed to swap over is made re-pop. It is just expensive as all get out for a good manual these days, and the flywheel, bell housing, shifter, it is endlessly expensive. I would get it running solid, then fix all the suspension and drive-train issues it probably has. If you find a 400- great- they are out there and will swap right in. An old school Pontiac V-8 is a marvelous engine. Then you make it pretty. That is what I am doing. Mine was an OHC 6 two-speed automatic. Long gone. Now I have a Pontiac 455, a Super T-10 manual, and a posi 4-wheel disc brake rear out of a '79 TA. Originality is not an issue with me.