Swapping a 4.3 TBI for another is easy, however swapping from a V6 to V8 is more complicated. Literally everything is different.
If doing this on the cheap, you will need an ecm from a similar truck equipped with the correct engine size (5.0 or 5.7), the same transmission as the one in your friend's truck, type of drive ( 2wd or 4wd ) and the same gear ratio.
Without doing that, you are destined for fail.
Yes, you can swap a different intake onto a Vortec to accept a TBI unit with properly sized injectors, and bumped up fuel pressure at the regulator. You also need the correct ecm, or at least a prom from a V8. ( someone correct me if I am wrong )
The roller cam and better flowing heads are going to require more fuel.
I am not certain of the 4.3 TBI injector flow rating ( # per hour @ XX psi ) but they are a different part number than the 5.0L, which is also a different part number than the 5.7.
You can not take your 4.3 TBI and jam it on a V8 with a random intake and expect it to run.