Going from a 6 to an 8 will cause mucho issues with smogging regardless of what year the 8 is from. Unless you like spending $100 a year paying some guy under the table to pass it for you. Or you move out of Clark County. I'd say find out if your motor is toast and go with a rebuild from a reputable shop. Many will include a warranty for 6 months or so if they are decent.
As a for a mechanic, try Performance Auto 565-1190. Ask for Mario. He's off Boulder Highway south of Sunset, next to the Skyline Casino I'm sure he's going to need to see it in person though, any legit mechanic would/should. But he's a good guy IMO, I trust him to work on my stuff. Tell him Gary Bodell sent you, just don't put it on my tab