Eagle Alloy 225's, 20". Do I need an offset?

[Eldubya]

Hey all, so on monday I'm going to bite the bullet and finally order these rims. Now I've already asked the backspacing question, and was told to go with a 5" on the front rims (20x8.5) and a 5.5" on the rear (20x10) now that part was easy but I'm not sure if I need an offset either? everything is stock on the truck (79 SWB Scottsdale) but in the future I will upgrade to disc brakes on the rear. Also I plan on doing a 3/5 drop on the truck as well. Thanks for the help, this just happens to be one of those items on the truck which makes me super nervous as i don't want to order them and they not fit when they get here.

As for tires I've been recommended to go with the 255/40-20's on the front and the 295/40-20 on the rear. Think that'll work with everything???

Thanks!