340 or 408?

I am another big fan of the 340. It is an amazing engine, well designed and very adaptable to upgrades. The 1970 340 should have a steel crank and rods. I believe you can stretch 2HP per C.I. if you really want to get extreme. The 275 HP rating is net HP and not gross so you are already around 350 HP with a stock 340, so pushing it to 500 HP is very doable. I would caution about going to much higher on the compression as 340s tend to blow through head gaskets. I picked up a set of T/A heads and a six pack years ago for my 70 Duster with 4:11 and a 4 speed, headers and a decent cam it made a formidable little street machine. The ford and Chevy guys hated getting smoked by a 340! Later I got the W-2 heads, a bigger cam and went back to a 4 barrel and ran her at the strip for a while with decent results with a 5:13 gear that I didnt really like that much. I am currently building a semi mild 440 for a 68 Dart but next year I want to get the Duster out of storage and bring her back to life. Stock 340 heads flow pretty good, the T/A and W-2s use mechanical tappets that are offset to allow more porting. Sounds like a great project you have there, keep us posted and HAVE FUN !