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Fuel delivery problem in a 1988 Dakota

I have an '88 Dakota, 230K miles, with the following problem:

For the last month or so, the truck has been hard to start--had to crank once, wait 10 seconds or so, then crank again. Gradually, this problem increased to the point where it would stumble at WOT, then even at 3/4 throttle or so.

Thinking it was the fuel pump, I replaced it, along with the filter. Problem remains; in fact, it seems even worse, to the point where the truck only idles (can't put any load on it, or it stumbles).

So, at this point I suspect the fuel pressure regulator. However, I don't want to buy it unnecessarily. Given that a
F.P.R. is designed to send a part of the pump output back, would pinching off part/all of the return line be a valid diagnostic test?

Anything else that comes to mind?

To check the fuel pressure regulator, turn the ignition switch on and off a half dozen times to cycle the fuel pump that many times. Remove the vacuum hose from the fuel pressure regulator and if gas leaks out of the vacuum hose replace the regulator.