Attempt at test running a Magnum

You say you've got it hooked to a '92 B250 fuel tank- is that the tank that was original to your engine? If not, that could be your problem.
'92 was an odd year, as the TBI LA318 was replaced by the 5.2 Magnum, but the TBI LA 360 was still being used and would not be replaced by the 5.9 Magnum until the following year in '93. Therefore, there were two different tank/pumps being used in '92 depending on which engine was ordered- the TBI system required about 14 1/2psi., while the Magnums with MPFI needed 55-60 lbs. IIRC. So, if you're trying to run your Magnum off of a TBI tank & pump, you won't have sufficient pressure for it to operate.