Mike i used the MP off set kit and it worked out to 3/4'' right on the money, The back space was 47/8" for the life of me i can't remember the off set. Sucks to get old........Ed.
Detroit is a way's away i'm 1hr east of Toronto,yes it has the tabs, I have a PM from a member that lives in Oshawa that i'm waiting to here back from.Ed
I ran avgas in the late 70's through the 80's in a high comp small block. Had 0 problems, In the 90's built a high comp 440 and used avgas in it.Tried some c12 and there was no difference in et, again you have to set your tune for different fuels.
If you want to run the biggest tire you can without doing a mini tub,Get some offset spring hangers, then you can run a 15"x8" rim with 4 7/8ths back space then you can run a 275 tire.
Stock wheel wells no cutting at all, 15"x8" rim with 4 7/8" b/s, i didn't like the look of the rallies
so i sold them and went steel rims, I got about 1 1/2" space between the wheel lip and tire and about 1" to the spring.And yes it's a Abody 83/4.