Trouble with 1983 chevy pickup

I drive a 1983 Chevy pickup. The original 305 V8 was replaced with a new GM 350 crate motor 2 years ago. The new engine has 19,000 miles on it. It recently started missing and loosing power between 45 and 50 MPH. I’ve replaced the fuel pump, fuel filter, fuel lines, distributor cap and rotor, coil, ignition module, spark plugs and wires. I have also tryed bypassing the gas tank by running the fuel lines from the fuel pump into a temporary tank secured under the hood. Last weekend, while at a car show/swap meet, I was looking at a new distributor, but it had only one set of wires entering from the side. My distributor has a second set of wires that plug into a 4 prong harness. The gentleman told me that what I had was a very specific distributor, made only 1 year that controlled the transmission shift and locking converter. Is this correct and could this be my problem?