Your fuel pressure is too low. You had better fuel pressure with the old regulator after your initial work. This indicates the pump is (or at least was, very recently) capable of delivering the required pressure. Based on what you have shared here, the original fuel pressure regulator does not appear to be malfunctioning. The newly installed fuel pressure regulator appears incorrect for the application or faulty. There are multiple options that will physically fit, but not all have the same pressure. If this was my problem to solve, I would reinstall the original FPR and check pressures KOEO and running. If good (58-62psi), the next thing I would check is the stock injectors. Does this vehicle have a hot start issue? I.e. after running up to operating temperature, shut it off and leave it for 15 minutes and then try to restart. Does it require an extended cranking time to start? If yes, your injectors are leaking, for which the stockers are notorious. If that truck sat for a few years the fuel in that rear tank is likely bad. and is being dumped into your front tank by the onboard tank balance controller. That may be the root cause of your low power problem.