Thanks to all who reply, I appreciate the insight. Due to availability and cost of finding M-block motors I'll likely be putting one in my '43 GMC hotrod project truck. I've thought over the different possibilities and the 335 series motors are the easiest to come by, and resonable to build, though I am fully aware of their faults as well. Still I think I'll go M-block power anyway. I really have 3 questions, first I have a set of headers my dad had on his 70 Mach 1 which had a 351C, any chance they'll fit the M or 400? 2, I know they are similar but what is the actual pheasibility of swapping both manual and automatic trannies between the 385 and 335 series motors. 3, I know the 400's have larger main bearings but what are your opinions on which motor will be most durable between the 351M and 400? Around here it seems that 351M's are quite a bit easier to come by, and right now I have my eye on a '78 dually service truck with a 351M 4spd. The motor is seized but if it isn't rebuildable easily I'll just throw the motor away and keep the tranny. But if the 400 is a much better engine, I'd be more inclined to go that route, and if a manual tranny from a 460 is a bolt on I'll steer clear of the old service truck and look for a running engine and a ZF from a 460.

 

The 400 is a much better engine than the 351M. The 351M in my opinion was an afterthought just produced to fill a gap in production.

 

first question is the headers if the 351c was a 4v engine, and you are going to use the 400 heads you will have exhaust leaks as the headers will bolt up but the 4v heads had alot bigger ports (351c2v headers will bolt up, and work on a 351m/400 usually as long as they didn't hug the block very close) 2 the transmissions from a 460 will work on a 351m/400, but not on a 351c

 

Torque1st, what about the 400's is better aside from HP and torque? I'd just like to know what the actual differences are between the M and 400. Monsterbaby, dad got an auto 2bbl, he always wished he'd gotten the 4spd 4bbl with a factory shaker scoop. So the headers I have will work, and a tranny from a 385 series will work as well, looks like I have some thinking to do...

 

the headers will bolt up to the engine properly, but why do you think mustang headers will fit in a '43 truck chassis? there's always a chance, but not likely. Cleveland headers on an M will finish out at the collector a little higher in relation to the chassis because of the deck height difference of the two engines. i think the difference at the collector is about 3/4", add to the mix that you are starting with car chassis headers into a truck and good luck! it'd be nice if they fit for you, but don't be surprised if you end up needing something else. there should be some M/400 headers for a truck chassis available from some maker? i'd say there is a much better chance of those fitting than the mustang headers.