I have a question and need your advise before I.

I have a question and need your advise before I start spending money on mechanics to give me the run around. Please bare with me as I try to give as much detail as possible. I have a 1989 chevy caprice with a 1995 chevy engine converted to carburetor and mechanical fuel pump with 1998 vortec heads. The transmission is a 350 turbo with shift kit. The problem is it fires up fine but when driving and speed reaches 45-55 mph the car loose all power as if it cut off. The strange thing is the car do not cut off but petal to the floor equals no power just stumble sorta like running out of gas. For some reason if I ease off the petal and allow the rpm to come back down the car runs fine until I reach the speed of 45-55 mph again. I have noticed a small tap sound like a lifter when engine first crank while cold but once ruuning it stop. I noticed this engine distributor do not have a vacuum advance nor is it hooked up to a computer just plain hei no vacuum advance. The oil looks good and no noticeable smoke out the exhaust while idle or drive. I do feel a slight hesitation while idling but not enough to complain about. The plugs and wires are new. The exhaust is is dual 2/ 1/2 inch with headers and dyno max super turbo mufflers. Any information you can provide to give me a starting point on what to have checked out would be very helpful and Thankful.