I am STUMPED! Strange problem in 440/120 circuit

[Chevy3+3]

I will try and make this as short as possible but really need to go into detail to show what I'm dealing with. My truck is a '88 V30 454 tbi.

First of all a couple of months ago I installed 2 brand new Delphi fuel pumps and a new fuel tank selector valve. All has been good and I have been alternating tanks every time I fill up.

Last weekend my ECM B fuse blew while running the passenger side tank. I replaced the fuse and when I tried to start it, it blew again. I switched to the drivers side pump and replaced the fuse and the truck started and ran fine. I drove it for several days on the drivers side tank and then my ECM fuse blew again with the truck at operating temperature on the highway.

I crawled under the truck along highway and unplugged both fuel pump pigtails and replaced the fuse. When I turned the key on it immediately blew the fuse on either tank selected. I had to be towed home.

The next morning the truck was cool and I replaced the fuse and turned to key to the on position and the fuse did not blow. So I reconnected the LH fuel pump and started the truck. Truck ran normal. When I switch to the RH fuel tank it blows the fuse immediately.

So here's a short list of where I'm at: Scenario 1-3 the truck is not running.

Scenario 1. Key off, LH tank selected on switch, install new fuse, both fuel pump pigtails disconnected, fuel tank selector valve disconnected

-Turn key to on position (not crank position) and fuse holds
-When I switch to the RH tank it immediately blows the fuse

Scenario 2. Key off, RH tank selected on switch, install new fuse, both fuel pump pigtails disconnected, fuel tank selector valve disconnected

-Turn key to on position and the fuse holds
-Switch the tank selector switch to the LH tank and the fuse remains holding
-If I switch the tanks selector switch back to the RH hand then the fuse blows

Scenario 3. Key off, RH tank selected on switch, install new fuse, both fuel pump pigtails disconnected, fuel tank selector valve disconnected

-Turn key to on position and the fuse holds
-When I engage the ignition switch to the crank position the fuse blows in about 1-2 seconds

Scenario 4. Key off, LH tank selected, install new fuse, LH fuel pump pigtail reconnected, fuel tank selector valve connected.

-Truck will start and run like normal.
-This combo did leave me stranded a couple of days ago when the fuse blew. I'm worried it will blow again eventually I just don't know when or why.

I'm truly baffled by this. Can anyone make any sense of this?

I know the oil sending unit could be at fault but not sure how it would distinguish between LH or RH tanks. I know my truck has a Hot Start fuel module somewhere under the dash but not sure if it could cause my problems.