For now I never switched belts and havent had a problem with slip with 3.5.If you go 3.4 definately change belt.Not sure HP increase but you will notice right away.

An explanation of what he'* talikng about......

I did the pulley swap on the weekend if my fiancee'* birthday. Having never done anything like this,it took me most of the day especially after ZZP had sent me the 660 belt,which is for the GTP. It was getting late in the evening and my fiancee was getting pissed since I hadn't spent any time with her on the day she took off for her birthday. I went ahead and put the stock belt back on and fired her up. MASSIVE BELT SLIP!!!!!!!!!. I had no choice but to leave it on and go ahead with the plans for the evening. The next morning after pulling the belt off,I wound up calling Boosty about it and we came to the conclusion that the 715 was the right belt. Fiancee was gone to work so,I called Advance and asked if someone could bring me the belt sincei live only abour 1 1/2 miles away,They couldn't do it. So,I wound up WALKING to Advance to get the right belt

I got it on and that'* the "Rest of the story"

715 belt for a 3.5 pulley ?

Sure... might be a lil tight at first...but it'll work out great.

Scott...that sure seemed funnier at the time.

Maybe to YOU!!!!!!!!

I thought the GTP had the same engine why is the belt different.

Coil packs are mounted differently due to being in the h-body car where there are no upper motor mounts. Therefore one of the idlers and I believe maybe the tensioner is moved a little bit (since they are mounted to the coil pack mount) requiring a longer belt.

Hope that helps.

Ya thanks just curious.

so wait i could just put the 3.5 inch pully on a stock belt for now?

Well, some people and I have without slippage untill you get a proper belt.Having said that , yesterday I bought the 715 belt just incase ,and I dont feel any real difference.Your best bet is to buy the proper belt You may not have a problem until you do.