we replaced my 1996 Chevy Silverado 4x4 truck engine with a new crate long block

There is a sequence of events when it starts. Somewhere along the line, the voltage drops to the pump because of another problem.

I have seen this before, and someone will respond soon with what it is. Maybe oil pressure sender, or something like that.

We replaced fuel pump. So all good on that now she wants to run at 2500 RPM (we are looking for a vacuumed leak). But the only thing we haven't done is replace the injectors or spider. Only running on 4 cyl

How do you know only four cylinders?

Can you check spark on each at plug end?

Because after initial start and run for a min you take a old empty windex spray bottle fill it with water n spray the exhaust manifold an only four of them instantly evaporate. Hence no heat so not running. It's a pretty neat trick. An yes we tested each plug for spark. All are working. Found a local that has another spider we may just replace the whole top end. We were wishing someone had a simple answer or idea.

I like the spray bottle idea. Sorry I have no ideas. Spider could be the problem, but if it works to begin with, wonder why it would just stop. How about ground connections? Maybe the vibrations of the engine running is causing intermittent connections.

All connections are good. Found some unfavorable items in the fuel tank when we replaced the fuel pump so we are figuring injectors. I have really classy neighbors in my neck of the woods so by unfavorable I mean hose pieces, 3 ft of garden hose. A piece of wire. Etc. my mechanic is my Dad he's been at this for round 50 + years, owns his own shop/business. But this ole girl threw him a curve ball. Just wondered if anyone out there knew a trick to our Delma!