Shortening a rear for the 66 Dart.

Here,s what I,ve decided.I,m gonna get 15x10 rims with a 4 1/2 backspace.I,ll need to take 3 inches a side of the rear(52 5/8 inches)will bring it down to 46 5/8,I,ll order new axels from Strange(25 inches)regualar A-body axels are 28 inches.My question is,what is the offset on the A-body 8 3/4?A guy that owns his own 8 3/4 jig is doing the shortening for me and wants to know the offset.Thanks.Any help will be appreaciated.Thanks

P.S I,m using 10x 2 1/2 inch brakes.