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.