1979 Big 10 5/7 drop input...

[Moose62]

I bought a 1979 Big 10 swb and I would like any insight on the best route to get a 5/7 or 4/7 leveled look. Because of the H.D. Suspension I'm going to lose the coils and leafs and prefer a flip in the rear with maybe some stock leafs from a c10 (if anyone has any willing to sell). What are the best products for the front to get a proper 4-5" drop?
Also if anyone has any backspacing info for a 20" wheel, 8" and 9" wide. I plan to run a 275 or 265/45-20. Front and rear same size.