I know you said it idles beautiful. By what I’m able to see and hear, it sounds like a misfire to me. There’s a difference in a free wheeling engine and one that’s loaded. I’d go thru and verify firing order and wiring to spark plugs. Also make sure plug wires are pushed on cap and plug all the way. Then personally I’d take my timing light and check each wire to ensure the timing light flashes consistently on each wire. I’ve messed up plug wires and crossed them to where it’ll seem to idle good with no back fire but will miss under load. You didn’t mess with trans other than linkage so go back to what you did and recheck. There are a lot of variables that have been added. New plugs and wires being a couple. New doesn’t mean good.
