I compression check when fresh. Then I have a known number. Same with springs. Get a on the car checker. They're not perfect but they will show a change. With these two tests you'll have a pretty good idea if there was something in the motor slowing it down. With the heads diasembled you can get a pretty good look at the seats and valves. If it compression tested good they may be fine. I'd send the lifters in. Inspect everything very well. It's nice to have the original build sheets to see what changed. Check pistons close for loose ring lands and cracks, pin for feeedom of movement, rod bolt length if measured originally, cap mating surfaces, cam lobes, valve lock grooves, diassemble and inspect oil pump.
Doug