IsaLandr, on mine a 40" was perfect for the rear.
20" is too short for the front, I used a 30" cut down to about 26" and re-did the double flare.
As far as why replace the front, my guess would be to match line and fitting sizes, I believe the stock line was 5/16.
Clover compound is a brand of lapping compound commonly found at good auto stores, usually a double can with 2 grits.
As far as spacing out the front, that is simply to clear the casting boss for the front head bolt.
Just use a nipple and a 90 degree instead of just a 90 deg by itself, gets it out just far enough to clear the head.
In my opinion the mod is worth it, faster start, smoother idle, maybe a litlle better botom and mid, just seems to run out smoother.
Hope this helps.