LS Engine - 1968 4-speed?

[Hey Dude]

My search didn't produce what I wanted... Has anyone here mated an LS engine to their original manual tranny? I have a 1968 C20 with the heavy duty 3-speed+ granny low transmission. Is this possible? My donor vehicle is a 2003 silverado 4x4 5.3LS1 My initial plan would be to convert the LS to a carburetor.

The C20 has only 35K original miles. Bought new by my great uncle but, the motor has sat too long in storage and needs valve work and a new carb. To turn this into a parts chaser and cruiser would be the goal. I came across the 03 silverado cheap...