Bolt pattern, back spacing?

you measure diagonally from lug to lug? Also what would be the correct backspacing for my 68?

Here's a really good article on how to measure backspacing..

http://www.mrt-wheels.com/boltpattern.html

For back spacing, lay/hold a straight, flat edge on back of rim, measure to the mounting surface on the back of the rim, subtract the width of the rims' lip.