Fuel tank pressure
 

Long shot but here goes:

90 chassis, all engine bay evap stuff removed (line is connected to fitting on the front passenger side of the subframe). Fuel system is walbro 450, fuel lab fpr, single pass fuel rail from flyin miata, engine bay is all an6 lines / fittings.

Whenever I open my fuel system (via cap or tank access to replace a fule pump) my tank has positive pressure. This holds true if only closed for a day or multiple months. I was told that is bad and I should fix it...where would you suggest looking first? Thanks!

You could, I don't know, install the system that was there in the first place to solve that very issue (evap).

Vent the cap.
It should have a one way valve in it to protect against vacuum and tank collapse.
Make it a two way...

Now, a fully functional evap system is the correct way to solve this issue and if it's working correctly it doesn't cost any power at all.
It should only operate at light cruise and is VERY effective at reducing HC emissions produced by the car.

50 years ago rich folks houses ALL had detached garages as the cars stank to high heaven.
The evap systems that came to be 50 years ago eliminated this requirement.
Look at old fancy houses, you will see that I am right...