I'm building a 1979 360 based 408 with a small Comp roller cam with a single bolt. When I put the thrust plate on I have tons of room for the cam to move back and forth but when I tighten the cam pulley bolt I lose all of my end play and it becomes hard to turn. It's been a long time since I built a small block but I seem to remember a spacer that goes on the end of the cam but I can't find mention of such a thing anywhere. What am I missing? Thanks in advance!

When you say spacer are you thinking of the fuel pump drive eccentric that bolts on to the front of the cam also

When the cam gear goes solid against the camshaft, there should still be some clearance to the thrust plate. I'd assemble the gear, thrust plate, and cam out of the engine and then check the clearance with a feeler gauge. I'll bet you'll find no clearance due to something being machined incorrectly.

No not the eccentric. Problem is when I do the pulley bolt up I can barely turn the cam.

When I do that it has enough clearance to turn freely, and it has some wiggle, but I haven't measured it. It's probably a thou or 2 of clearance I'll go measure it. Hmm I might be looking at a different cam pulley.

I seem to remember a thrust washer you had to put on before the keyway.

The clearance is set by the back of the top timing gear, with the plate sandwiched between that and the face of the first cam bearing journal.