"Will a mid 60's/70's chev truck 4 speed bolt onto the existing 235 with torque tube 3 speed ?"
It depends on the 4-speed.

I think that the open drive-shaft SM420 4-speed was made up to 1967.

Other people will post information for higher-geared 4-speed/5-speed transmissions that will/might bolt up to you clutch housing.