weird issue with fuel tank pressure and venting...I think

[1972BlueC20]

I've discovered a new weird issue since getting my truck back from a new engine install. The shop that did the install is going to look at it for me and most likely fix it, but I'm hoping I can figure it out myself as I like tinkering with the truck, after all that's why I bought it.

Some of you might have been following my ongoing saga of issues over recent weeks, but the latest weird thing is puzzling me and I'm hoping you guys can help!

So I was having issues with dropping fuel pressure and air bubbles in the fuel filter after the truck was idling for a while. I noticed that my return fuel line was left open at the engine side but still hooked up to the tank. I also was running a non-vented fuel cap for the last 6 months with no felt problems (on old original engine).

I plugged the return inlet on the tank itself, and I installed a new vented gas cap.

The next time I started the truck it ran fine, but after I shut it off IT WOULD NOT RESTART! I tried a few times but nothing.

I decided to see if the gas cap was causing the issue and I went to take the cap off and there was TONS of pressure that released.

After taking the cap off the truck started right up!

I put the old cap back on, and so far it's starting and restarting fine.

Any ideas what's going on here??