please help eletro speedo signal to mechanical speedometer

[mycarismashed]

ok here the poblem is, i have a 1996 mustang gt, i blew my transmisison (t45) and switched it for a 2001 tremec 3650. now the pic ups for the tremec 3650 speedo gear uses an eletric signal and my speedomoter works on a mechanical signal. now i talked to tremec and they told me i cand just remove the electronic gear and replace it with the manual gear as well i cant switch my output shaft either. So at the present time my car will run with out the speedmeter ( the speedo is unpluged cause when i plug it in it starts off at 120 miles and then i hit the speed limiter and it cuts off the fuel) but when i shift into neutral with my speedo gear unpluged after going fast my car will just die on me. It does not bog down or stall out it just turns off. Does any one know how to correct this problem. Or does any one know what is causing my car to stall out. Please help thank you.