Hello i have a 89 fox 5.0 that i bought non running. I brought home and managed to get it running only to find out the block was cracked and mixing coolant and oil. Long story short i got a junkyard 5.0 out of a 2000 mountaineer took the heads off sent to the machine shop had them resurfaced and valve job as well as added better valve springs for cam swap. The old engine already had it so i just swapped. Put the engine back together with the gt40p intake new ford 24lb injectors new accel distributer upgraded altenator deleted egr system new 24lb calibrated maf. The car was an auto and i went ahead and did the manual swap also. So got the car timing set at 14degress and everything was fine. I started taking it out for test drives and it ran extremely well. It ran great for 2 days then after stopping for fuel the car started to spit sputter pop and bang under heavy load free idle it seemed fine. I got her back home and figured it was fuel related since it happened to occur after pumping gas so i decide to drop the tank and take a look the baffle was broke but the pump assembly seemed fine. I decided to go ahead and replace the tank as well as the pump thinking this was my problem. Also replaced my fuel filter put everything together and still the exact same issue. I replaced my plugs and checked gap. Still the same. Oh and i replaced my wires when i put the engine together so those are fine. I have a fuel pressure gauge so i decided to see what was happening without the vacuum line it would hold 40 and with it connected it dropped around 37. So then i started accelerating and noticed that under acceleration it would drop about 10psi to about 27 28psi. So i unhooked the vacuum line and capped it and checked fuel pressure again. This time it seemed to hold 40psi so i took it for a drive with the vacuum line to the regulator off amd capped and it ran alot better but would still have the same issues this time higher up in the rpm band. So i ordered a new fuel pressure regulator adjusted the pressure at 45 without vacuum 40 with but it still drops while accelerating no load. I havent test driven it yet but am sure its still the same. Im at my wits end with this thing i dont know what else to look for or check. Any help or guidance would be greatly appreciated.