96 K1500 Engine Replaced(Carb), no tranny downshift

Hello,
Hopefully somebody can bail me out once again. I bought a 96K1500 off of a buddy. He replaced the original engine with an older style rebuilt Carb 350. My problem is the tranny wont down shift when you put the pedal down. My mechanic told me it is because the throttle sensor isnt there and b/c it is a carb not a throttle body. Does anyone know of a way short of replacing the engine or tranny with bypassing my problem so the tranny down shifts when it is suppose to?

I am surprised it shifts up or down properly, the unit is pcm controlled, from engine load,speed, and throttle inputs, you have eliminated these functions by changing the engine, no easy way around this.

So best way would be to put in the correct engine, correct? I am going to show my ignorance here but could you tell me the difference between a SFI Engine which came in the truck and the Vortec? If i purchased a Vortec engine for a 96-02 it would fit properly correct?

Sfi is a different design than the " spider" type vortec, google either one plenty of info out there.