rear width

[Ozzy2013]

Guys need a favor ive looked all over and found everything but what i need .I have my 68 GMC 2wd all apart down to the frame .I have a 62 wide rear drum face to drum face stock 6 lug dana 44 rear .I found a 84 ford 9 inch im going to use .The question i need answered is how narrow should i cut this rear to get as large a tire as i can under it? It is a stepside and i dont really want to tub it .Im hoping some one has done this combo (sure some one has ) it would save me having to mock everything up as i have parts stored all over !Any help is greatly appreciated.Oz i will run rims with what ever back spacing works best for this application oh the 9 inch is 65 wide drum face to drum face .