My engine is playing up. It's a '78 350 V8 with HEI. The idle is OK, but when you rev it, it sounds rough, like it's not firing on all cylinders, and there is no power at all; zero acceleration. Feels like something is straining it, and it seems to be doing about 2MPG. One of the front brakes is rubbing slightly, but not enough to affect the engine this much. I recently rebuilt the carb (Holley), and as far as I can remember, I had tested the engine since reinstalling it (but did not drive the truck because I had no brakes), and it ran well. Sounded great and revved very quickly. I also recently fitted headers, new mufflers, new exhausts, new plugs, changed the oil, new oil filter, new air cleaner, and a new PCV valve, but the truck is now performing FAR worse than before, and something is very obviously wrong. The only fuel filter I can find is in the carb fuel inlet - a gauze type filter, but I cleaned that. I have not yet taken the truck for an engine tuneup/check, but I don't really want to drive the 20 miles or so with the engine running as it is. Can anyone tell me what the problem might be? Thanks.