71 burb reduced power

[MMricky]

I have 71 c10 suburban with original 350 , rebuilt, sort of. Not confident that it was done correctly as it still has white smoke coming out of tailpipe. It’s been running fine for the 2 years I have had it, recently got the oil changed and topped off trans fluid and now after about30 minutes of run time it’s like a switch I get reduced power. The more gas I give it it just wats to stall, oil pressure is good and trans shifts as normal just won’t go. Don’t know if it is motor, trans, fuel issues. Just can’t figure it out, it always starts, just happens after it’s run for a while. I have 700r auto trans with edelbrock 4 barrel, mechanical fuel pump, running no ethanol in gas, spark is electronic being no points, just not sure how to diagnose any help would be much appreciated