I'm measuring .018" exhaust and .040" intake lifter preload with Johnson lifters and new cam. They recommend .020" to .040", but with stock rockers, I really don't think chasing a couple thousandths with custom pushrods is cost effective, but is it necessary? The next set of off the shelf pushrods will put me at over .100" preload.

What type heads are you running?
If stock you should be fine, if aluminum not so much warmed up

Stock, whatever heads came in '71. Must have been rebuilt once, new guides, but smaller than stock cam, and extremely loose chain, retarding the cam 12deg. Put a 220/226 .475"/.494" Lunati, springs, and Cloyes chain.

Doo your new lifters have spring steel wire retainers or the Tru Arc snap ring retainer's in the anti pump up lifters?
If the snap ring or the stock type retainers you should be fine with that amount of preload on both, if they make noise change them to the .100 longer ones with the stock wire type retainers

I would get longer for the ex. What length are they? It shouldn't be to hard or expensive to get them.

why a loose chain ?

Pony up for a set of adjustable pushrods ... I doubt your valve stem heights are equal ... and set it at exactly what you want. Use the current length and add ..200 as your max length.

Pushrods are 8.50". I think stock is 8.575".

I guess the chain was just worn out. I didn't see excessive wear in the gears or anything that stuck out. It was just sloppy, and I measured the cam timing out of curiosity.

The exhaust valves are consistently higher than the intakes, but the exhaust has less clearance. I guess the base circle is ground down more on the exhaust to give it more lift?

I had some Comp Pro Magnum adjustable rockers I was going to use, but the pushrods were really too long for those, then I read that the geometry with the rollers required the shaft to be shimmed, and I figured what's the point, just use the stock rockers. I just want the thing to run and not blow up at this point while I'm building the real motor.

They're A-0976 Hylift Johnson from TopLine with the wire retainers.

those are the standard non anti pump up lifters which will work fine with more preload than the anti pump up lifters will
Have you check the preload on all 16 lifters, if not you should

I've only checked #1 and #2 with the light checking springs, and wondering what people think. Is there a way to check preload once the motor is together with the real springs and intake on?

Get some 8.550's