sluggish transmission

hey all, so i have a 1997 Chevy C1500 with a 5.7L 350. i’ve recently basically rebuilt the engine due to low compression in cylinder one (i had the heads and all the valves redone, while my engine was out i cleaned it up and replaced a few other parts). when i pulled the engine i also pulled the transmission. i never touched the transmission other then taking it out and putting it back in. i went for a test drive, i had a cylinder four misfire which if i remember right was just a loose rocker. now when i go to drive it i have a code for random/multiple misfires and my transmission is very sluggish. when i say sluggish i mean that it feels like im getting no power to my truck. it’s very unresponsive and won’t even move until it’s close to 1500rpm when before it was mush lower. my truck won’t even just roll on its own like it used to now. i would like to add then i did slightly overfill the transmission but ive drained it and its back at the proper level. it also feels like it’s having a hard time shifting. i drove it down my road, speed limit 30, and before i even hit 20mph i was at over 2500rpm. i used to never see my rpm that high unless i was doing close to 90mph. i’m not sure what’s going on or what could be causing my transmission to be so sluggish. a buddy said maybe it was the fluid being overfilled but i have drained and got it to proper fill level.

if you’ve read all of this thank you so much for your help in advance, im open to any kind of advice i can get. thank you all!