What gen small block is this ? How new is this carb ? How long was it sitting if not new ?

This is a bare stock 350 motor, don't see how reversion could be an issue

It's all stock basically, only mod is a 4175 vac secondary All dodge. I think a gasket leak or pinhole could cause reversation even with stock cam etc.
I started it before work and cranked fine and ran little grumpy. So advanced throttle to like 1200 when it was chugging and turned idle mixture screws in. They typically at like 3/4 turn out and are very senistive. Like a couple degrees changes the speed a lot. I turned them both in till seated and although it didn't want to idle well at all it didn't stall. That seems weird it would run at all with both of them fully seated.
More history, Carb is about 10 yrs old and hasn't been apart since I installed it and did some minor main jet changes with old motor. Didn't sit long as it ran fin before the old motor head cracked. Also, I looked up my maintance spreadsheet and manifolds and risers are about 3.5 yrs old, I thought they were newer than that but they looked ok. ( I know appearance of the passages is not the total picture though)

even a bones stock 350 can revert water if it has an exhaust manifold with an internal crack that allows water to enter the exhaust stream.

I'd do the simplest and easiest first. Sounds like you could be loading up on incorrect timing. YES, backfires and Holley PVs do NOT like each other. Although the newer PVs seem to be more resistant, they are still vulnerable to backfire. Do you have the right one in there in the first place? Holley PV are marked on the flat side with a number, which is the Vac load. You need to determine your vacuum at idle and make sure you have the right one in. I'd change PV (for fun), set float levels, reset my idle screws, Check for vacuum leaks (double ck to make sure you covered all non-used ports--don't ask! LOL!) double check timing and take it out and have a beer before I'd get all tied up in reversion and all that complicated stuff. If you know for proof positive the fuel and spark are right, and you still have the same situation, THEN start looking around at other stuff. JMHO, I could be wrong!

Longer than 5 or 6 months ?
I'd still go in and give everything a good cleaning. Carb cleaner and stick red straw in everything while blowing out both directions. Don't forget to remove idle mixture screws and clean thru all (4 each side) air bleeds. Blow out with air gun just after. See if there is a fuel filter in the carb inlets - some of these have them. Check needle and seats...I've had plenty with ust a few years use have bad o-rings and or tips on the needles.

Cap and rotor okay ? No, I don't care if fine last time it ran. LOL.

You didn't answer what Gen motor this is,so, does this motor have roller lifters or flat tappets ?

What's your initial and total timing ?

Has the subject of a collapsed lifter come up yet?

I ordered a rebuild kit for carb from local speed shop and pick it up tomorrow. Carb only sat about a month while I was doing long block. It's a Vortex with roller lifters. ( 1999 was original motor). Timing is 8 deg btdc, at least that's where I put it when I dressed it. I can't recall what the total is, it is the stock preto lite distributor with weights,,Cap and rotor have about 100hrs on them. When I got home from work pulled spark plus out and they were fairly black sooty and smelled like fuel. No water on them at least best I could tell. PV is currently a 5 and plan on replacing it with that but may buy a couple others when I am picking up rebuild gasket kit.

why did the old motor crack a head,did it freeze or overheat?

I assume it just had eroded water jacket, it was raw water cooled and had about 1600 hrs of salt water use. Head got a crack I. Exhaust port right around the boss where the valve guide pokes thru the exhaust port.