For what it's worth
Okay, here is what I have and it's no where near conclusive, as I am still working on it. I have a 20x8.5 (9.5 inch total width) rim. I have a 66 GP and the offset of the rim is +14mm (approx. 1/2 inch). I figure that the backspacing is approx. 5.25 inches with this setup. The rim has a 255/50 R20 tire on it currently (which doesn't buldge that much). With this setup I have 1/16 inch in the front by the wheel well and 1 15/16 on the rear to the frame rail (car is not being driven, I am going through it and making it mine). The shock tower is not an issue as there is plenty of clearance with this size rim.
I also have a 65 Full-size posi rear that is going into the GP. From articles I have read the 65 full-size axle is 3/4 inch shorter on either side, although all the mounting is in the exact same place for 1966. The way to tell it is a 65 compared to a 66 is to stick two fingers in between the backing plate and the lower control arm (?). If two fingers fit it is a 1966 rear, if they don't it is from a 1965. I am hoping this is true because I am looking forward to the rim being recessed 3/4 of an inch to give me more of a centerline. If the rim is recessed the 3/4 of an inch then I calculate I can get a 285 on the rear, but (and this is a BIG but) I will have a little over half an inch on the backside of clearance and a little under 1/2 inch of clearance on the front. If I stick with a 275 tire I will have over 1/2 inch on the front and the rear with this combination. The tire would be a 275/45 or 275/50 R20.
Sorry to be long-winded but I truly love this site and it has provided me with an exceptional amount of information that until now I have not really been able to contribute much to.
Does anyone know how much clearance should be maintained on the outside and inside at a minimum? I am planning on using Poly bushings to limit the amount of axle travel. My friend, who knows Pontiac's pretty well, said to leave atleast 1/2 inch on either side.
If anyone wants I can post some pictures, just tell me what you want to see pictures of.
Thanks.
Larry "Kid Skid"
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