As said by a few the big prob with your motor is the low compression and that cam. first thing to change is that. next is the exhaust. Free it up let it breath, very important. yeah the trans sucks but you need to make the engine run right first. Change the cam and exhaust get the distributer curve right and set up the carb. the 400 is going to use some fuel but it should not run like a pig. unless you want to start with a whole new engine that's what needs to be done to the 400 to make it right.
A few have mentioned timing. Distributor is 'new' (reman) one light spring, one heavy. 18 initial and 38 total. Has vac. advance, did not verify how much the vacuum is adding but the canister is new and works (verified with vac. pump). New Champion #14 plugs, new Taylor wires, new rotor, MSD Blaster 2 coil, new engine harness, ballast resistor. Before I changed the carb to the 600, the plugs were a nice light brown and the firing ring was right in the middle of the bend of the ground strap. It's right or at least as right as it could be with the current combo. Timing is not the issue.
If I stick with the 400, the cam will be changed for sure. I guess I'm just trying to figure out whether a cam change alone is worth the effort without doing anything else?
Full dual exhaust in these trucks can be a little tricky with the gas tank along the driver side. I'm sure it can be done, just another hurdle to contend with.
