Nice looking car, It's nice to see someone else doing an early A Pro Tour. One trick I did was to duct tape 2 pieces of 1" thick wood to the outer chassis rail to rest the tires against, so I knew I would have 1" of spacing on the inside. My rear wheels are 18 x 10 with 6" BS, the offset is included in the backspacing ( my wheels are 10" wide with 6" BS = 1" positive offset, because if you moved the mounting surface back 1", my wheels would be 5" + 5" = 0 offset) check out this link;
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101.
If your wheels are 10.5" with 6.8" BS that would give you 1.55" pos offset. So if we add 1.55" to my dimension of 53.5" gives us 55". And like I said earlier, my rear probably should have been a 1/2" wider, ( I have plenty of clearence to the frame, the spacers would only be for apperance) so add that to our number and we get 55.5". Add 1/4" to your number to be safe, 55" wide.
You do what you think is best, but if it is too tight to the chassis they will rub whenever the body leans. So TRIPLE check those numbers... Good Luck.
