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.