So I went back through and reset preload on lifters with no noticeable change. I bought a new Edelbrock 1406 and with this, issue is much better. Still not what I would expect. Again, I;ve had 5-6 mustangs with similiar power and ran much better. Most of them heavier cars. I may have found the issue though. If I accelerate hard (or try) I get somewhite smoke from exhaust. May have a head gasket leak? Have not noticed any changes in oil, like antifreeze getting in. But I think this would cause power loss and the smoke? Correct. Guess these second hand heads could be issue. If I tear it down, those suckers are coming off for some aluminum ones. Was able to take to first care show NRMA at Beech Bend in KY. My 9 yo loved it!
Until you have hard data, like compression check, you’re just chasing the symptoms, IMHO. Also, if you have access, you could check the tempature of the exhaust at each cylinder. Knowing both of those data points narrows the potential culprits. Keep us informed.