As the title says, is it normal for exhaust to suck air?

Some background iv got a stock 350 vortec crate engine from jegs with a tbi and a 7427 computer controlling it along with true dual 2.5 inch exhaust pipes with an X-pipe midway back. Iv been facing some rough idle issues with the truck the whole time iv tried tuning it and its very frustrating.

At idle with a napkin over one of the exhaust tips it gets sucked in pretty quick and spat out over and over. Iv read that this is exhaust reversion but is this normal?

When doing the engine swap i broke an intake bolt and had to take the driver side head off to get it out then lined everything back up and torqued it down. My buddy said it should be fine since it had never been ran, so i never checked anything with a feeler gauge, just put it all back together.

I have not put many miles on the engine, less then a 1000 so i know it hasn't been broken in yet.

Im still new to tuning but i have the computer set up for open loop and have the tables set accordingly to the AFR i want to run at idle. When it runs rough there is no fluctuation in the wideband at all during idle. Im no expert but i want to say timing is causing the shaking of the truck, but im also not sure if its something mechanical instead from putting that head back on. When i first start it and its warming up however the idle is fairly smooth until i drive it and stop at a light then a switch almost flips and it starts idling bad and shaking the truck.

Hopefully you can make some sense of what im saying. Im just to the point im having more failures then success getting this truck to run good and be reliable and its getting overwhelming and costing money.