it's been starting rough and stalling out usually after just a couple minutes?

Hello, so I'm trying to help my friend with his van that's having some issues right now. I just got a look at it today and I'm trying to get a better idea of what to check out next. I'm not very familiar with the TBI system. The main issue he's been having is that it's been starting rough and stalling out usually after just a couple minutes of running. It seems to sound fine when it's running. When I checked it out today, I first noticed that he had a bad battery and a broken ground (smaller one coming off the main battery one) so we fixed both of those issues. I had him turn the key to the on position and I could hear the fuel pump relay working and hear the fuel pump running. It seemed to run for a long time, maybe 7-10 seconds with the key just in the on position no cranking. The van started right up first try after replacing the battery but suddenly stalled out after we let it idle for about 2 minutes. The van also has an interlock that was installed about 2 months ago. At this point I think the issue might lie in the fuel delivery because it does start most of the time and usually stalls out after a while. I want to check the fuel pressure next but I'm not sure exactly where to tie into the system since there's no Schrader valve and I don't have one of those special TBI adapters that replaces the fuel filter. Any advice on stuff to try next is very appreciated. I rented a basic fuel pressure test kit and I'm going to look at the fuel filter tomorrow.
Thank you,
Ella

Van listed above has a 5.7l V8 TBI.