If you're running ethanol blend fuel there is a good chance you may start vapor locking with the mechanical pump. You may run good with a mechanical pump, but you may run better with an electric pump.
Even Dwayne Porter had been trapping around 119 for years with his stock head 383, which seemed impressive enough as it was, and I think he picked up 2 MPH when he went to an electric.

I had a carter 6903 on my car, and it ran good cold but heat soaked it was a dog. Fuel pressure dropped from 7 to 3 in first, hovered down around 3ish in 2nd and 3rd. Didn't nose over hard, just ran weak.

If you stick with the mechanical run a vapor separator fuel filter, I wish I thought to try it before I bought my electric pump to see if that solved my issues. If you can verify your fuel pressure with a gauge under load.