Swapping a V8 for a V6 won't be cheap or easy- - - -the bellhousing bolt pattern on the V6 block does not match any V8 I'm aware of, so you'll end up fabricating mounts both front and rear, and swapping the bellhousing, adapting or fabricating clutch linkage, etc. On the bigger rigs like yours, you're probably going to find the transmission has the full round SAE-style bellhousing flange. Those housings are available for all sorts of engines, gas and Diesel, but they're scarce and expensive.

I'd suggest dropping the oil pan and confirming the source of the knock. The GMC V6 engines are pretty pricey to rebuild, and they're gas hogs, but once one is rebuilt properly it's a long lasting, high torque engine, and it's tougher than a two dollar steak!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!