I want to replace the tired and knocking 318 in my 84 Ram 4X4. I have a late 71 340 and 2 sets of heads. Js and Xs. I may tow the GTX a few places and obviously want a little more power and torque. Nothing drastic. I know a 360 might be more logical, but I have the 340 in hand, and it's balanced the same as the 318 so no torque converter differences, plus the truck pan off the 318 should also work on the 340, right? The biggest kicker is that I want it to live on cheap 87 octane gas. What's the max compression I can get by with? 9 or 9.5-1? I'm figuring a pretty close to stock overhaul. Maybe Keith Black pistons, a mild Comp cam and springs. I don't want to run a thermoquad, but I do have a clean 71 440 Carter AVS in my stash. How would it work if I could find a square bore manifold? Also, are there any differences in 318-360 truck exhaust manifolds? I really don't want headers, but I do want a decent flowing dual exhaust.

the oil pan as well as the manifolds will work on the 340 motor, i just droped a 73 340 block in my 78 pickup.

I'd sell the 340 and build a neutral balanced stoked 360 with relatively low compression and a mild cam. That's your best bang for the buck.

If the 340 didn't need a rebuild then I would just put a stock 340 cam in it (if it doesn't have one already) and run it. The only problem is that a 71 likely has too much compression for today's pump gas.

The 340 is apart and of course dirty and likely will need boring and the crankshaft reground. I may dig it out and at least have it cleaned up and checked. The casting date is 9/71 so it's likely a 72 production low compression engine.

The problem is that with their lousy open combustion chambers, the Mopar V8s tended to have octane requirements like their advertised compression ratios.
R.