The 4.3 engine is actually a 5.7 engine that has been cast with only 6 Cylinders.
The internal components (pistons, rods valve train) are all the same parts.
All of the accessories on the front of the motor will bolt up to either block.
That being said, The motor mounts that worked for a 5.0 or...
I see a lot of parts discussed here but no mention of the transmission.
Check that the TV cable is hooked up and adjusted correctly. the lack of shifting at anything but high rev might indicate a problem there.