Looking for guidance/poor power/shifting
 

69 Camaro with a 350 engine and a TH350 tranny. Running extremely well then one day I started out and it had really poor power, was surging, and really labored trying to run thru the gears. Checked timing, plugs, wires, fuel supply. I checked for antifreeze in the oil/oil in the antifreeze and all checked out fine. Checked all the wiring, checked transmission fluid level and all checked out fine. Nothing else to do on a Saturday morning so I pulled the intake and don't see any signs of cracks or leaks. I'm thinking I should pull the heads and check for cracks, check the valves, pistons, cam etc. Easier than pulling the transmission at this point. Looking for guidance/recommendations before I get any deeper into the engine. Any help would be appreciated.