I have great compression on 5 cylinders and
30 psi lower on the other 3. I know this is (ok)
but thinking maybe a head gasket or something?

Cyls 12468 nice at 180, cyls 357 only 150-155ish.

Most likely not a head gasket. You'd have other problems if a head gasket was bad. Did you do a leak down test? That will sometimes give you an clue as to where your squeeze is being lost.

No, not yet, it's on my list of to do's. It runs good, just thinking i'd like a little closer numbers.

did you do the test hot or cold? all the plugs out? WOT while cranking?

IMHO you are chasing a ghost .

There's nothing wrong with the numbers. Mainly because discrepancies in the test can make a huge difference in numbers even on a good engine.

Was the motor up to temp?
What was the battery state?
(on a newer application)Were the injectors disabled or not on to prevent fuel washing?
Were all plugs out at the same time?
Was the throttle at wot?
How fesh is the motor?(rings might not be fully seated yet)

That's why the factory specs are so vague. As long as the engine is running properly, it's all good.

It's a 340 with a M1 intake and edelbrock performer RPM cam. Motor is new, say only a few hundred miles.

All the plugs were out, fresh battery but the engine was cold as I just put it back together after dialing in cam timing. (long story)

The only error I realize I have done is the throttle was not at WOT. Lash on valves seems ok but will double check that also.

Where can I get a leakdown tester that will not break the bank?