Will not start

Out of curiosity, what is the fuel pressure reading with the key on, and engine off? These trucks need a minimum of 60psi to start. 58psi will not do it. 59 might do it, but probably not. They're very finicky about fuel quality and pressure. Also, have you changed the fuel pressure regulator yet? A bad regulator can cause all sorts of weirdness. Here's our set of tests for the pump and regulator: https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator?

Also, is the service engine light on? A trouble code there could easily be the problem. Let me know.

Ugh, those spider injector setups are a bit of a pain. And they're a common problem in the mid-90's S series trucks. Diagnostics can be tricky on them, but here's a how-to guide on YouTube: https://www.youtube.com/watch?v=AAfr9KupSVM

And here's a write-up that I like much more than the video: https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/testing-the-spider-fuel-injector-1

Definitely check the spider. Also, are you sure about those fuel pressure readings? Because the idling fuel pressure should be 5 or 6 psi less than with the key on, engine off, and not 5psi more, like you stated above. If it's going up while idling, you've got some sort of funky thing going on in the fuel system, probably the regulator (which you already replaced). So that's a little befuddling to me.

Finally, there are two different 4.3L engines in 1995. I think the differences are in the injection mainly. Would you mind posting your VIN number here so I can look up which one you have and possibly be able to guide you more accurately.

You might also try getting another fuel pressure gauge to put on it, since it's reading so high while idling. There may be some sort of issue going on there. For some reason I'm thinking that has to be related to your issue, and if we can hammer it out, we'll have it starting and running like it should.

I also wanted to ask if you've replaced the fuel filter since putting in the new pump. Generally, it's strongly suggested that you do it at the same time as the pump, because sometimes a failing pump can kick up some junk and clog up the filter.

As for the Vortec itself, the core of the engine is about as rock solid as they get. But those early spider injectors are somewhat problematic and so are the fuel pumps. My 2002 Blazer is all stock, with 207,000 miles on it. And I use it to run rural mail routes four to six days per week. I beat the ever loving crud out of it, and it just keeps going. The engine (essentially the same as yours, with some updates) sounds like it has about 50,000 on it, and the transmission still shifts about that smoothly too, And trust me, I do not baby it. Stop and go up to 650 times in a day. I have been through a few fuel pumps though :D. I'm on pump number four right now. Thank goodness for lifetime warranties.