Transmission? Poor acceleration.

[telly]

Changed the PCV valve today. It was sludgy and didn't rattle when shaken. I then ran the truck for about 20 minutes and kept watch on the white/gray smoke coming from the exhaust (trying to burn off any excess oil that my have blown by while over-filled). After the white/gray smoke greatly diminished, I took the truck for a spin "around the block" (actually about a 3 mile loop). I don't seem to be having transmission troubles, at least no noise and/or clunky shifts. And, I still don't have any oil leaks that would indicate a main seal issue.

When I replaced the plugs, I did so one by one...remove plug wire, remove plug, insert plug, replace plug wire. I did not gap the plugs...I used them straight out of the box. It is HEI and when I reset the timing I did so with the vacuum advance unhooked and plugged and advanced to 12. I chose to reset the timing as I was getting some backfire upon deceleration...there is no backfire now after the reset.

The fluid in the tranny appears to be fine...not dark, although I haven't yet dropped the pan to see if there is any metal shavings in there.

*A bit of additional info...the truck will currently not spin its tires from a stop if traveling in a straight line, but it does have enough power to spin them when leaving a stop and turning onto another road.

**Another bit of additional info...I believe the engine to be a 350, but I can't be sure. I bought it from a guy who bought it from a guy...that guy supposedly built engines for racing and threw this one into this truck when the person he built it for didn't pay. The guy I got it from couldn't remember or didn't know what block was in it, and had no idea if it had been bored at all or what year. I haven't crawled back into it to take the numbers off the block. It may be a later 70's block, but I do believe it is a 350...just not certain that it's still a 350. With that information, what would you suggest the plugs be gapped to?