I have a 79' monte that i'm rebuilding and I have a 425hp 383ci and new th350 for it. I have the original 7.5 10 bolt in it now. Can I swap an 8.5 3rd member into it or do I have to change the whole rear, drum to drum? I know that the drive shaft is 0.99" shorter on the 8.5 I'm just looking for the simplest switch.  

Yes you do have to get a complete setup,Currie and others have rearend packages to put a Ford 9" or GM 12 bolt there appox. $2000 ready to install.  WIEDER  

Yea, I saw that @ the Jegs web site. It looks like $2,443 before shipping and the brake package. I was just trying to save some money. I know its worth it and I have so much invested already that it would'nt make sense not to go all the way, I was just hoping there was an easier way, thanks.  

You could look at using an 8.5" 10-bolt form a 2nd Gen Camaro in your G-body Monte, or possibly a 12-bolt form an earlier Camaro. The other option is a 12-bolt from an A-body, you'll need to probably have it narrowed and the spring perches moved, but that'll still be cheaper than buying a whole new rear end unit.  

Even cheaper than trying to find a 2nd gen f body is the S10 rearend that have the same dimensions as 1st gen f bodys, and they re a hell of a lot easier to find.  

I do believe that S10 rears are all 7.5 and  7.625        WIEDER