renix probs

i just recenly purchased a 1989 renix to later figur out it has a motor swap with a 1994 4.0 ho motor but they use the renix intake and exhaust and im having a fueling issue low fuel presser when it get abouve 160 degrese cold it runs fine changed fuel pump and regulator with used parts but no diffrence ahelp please

How low is the pressure dropping?

how do you know it's a fuel pressure issue?

Something like that would be constant if you had a fuel system problem. The fact that it happens about 160 is a tell. That's when the Renix ECU goes closed loop.

What's the O2 sensor voltage like?