1988 Mustang GT

My pony has 119K original. We recently changed the spark plugs from split fires to stock. Since then, it's hard to start, cold or warm. Once it is started, it bogs out. If you accelerate, it makes a popping sound from the engine. We had the fuel pressure checked and it's at 20 psi. Our problem is we're getting conflicting diagnostics. One says it's the fuel pump, another says it's the regulator, some say it's the relay. Any hints? I'm leaning to the change in the spark plugs.

Which engine?

Also check the GAP and Ignition Timing.

It has a 5.0/302 engine.
I noticed a huge drop in performance after changing the plugs, this didn't happen immediatly after. The timing was set at 10 below top dead center. However, when the cap was repaced, the timing moved to 10+, but the mechanic said that was normal. I'm not so sure.