Sometimes the trouble you are having is from the fuel getting hot and vapor locking. If you still running the mechanical, you are still running the fuel through something next to the hot motor.

On the other hand, I'm running a Carter M6903 mechanical pump only on my 440 at the strip with no problems.

If you were to ask for a show of hands all the people running a good mechanical pump only at the strip, you'd probably get quite a few replies from people with way faster stuff than my high 12-second street car.