In the 1st gen cars, I know that all the I6 cars came with 4 lug wheels (vs 5 lug for the V8).

Does this also hold true for the 2nd gen cars (67&68)?

 

yes.

 

And the rear end on 6 cylinders - 7.5" ?

 

The rearends are 8 inch v8 7.25 for the inline6. Easy way to tell is the lugs all 8 and 9 inch are 5 if its a 4 its a 7.25 it was used for the inline 6 cars from 64.5 to 70.The 71-73 went to 5 lug only the 7.25 is still used today in the v6 mustang was also used in the ranger and aerostar buts been updated over the years.