Are you buying the 20's or is he giving them to you?
I wouldn't even consider swapping to 6 lugs if you have to buy the wheels, and I probably wouldn't even if they are a gift. If they are a gift, sell them or trade up for 5 lug wheels.
It'll be a big expense and pain in the rear and then you'll have a heavily modified truck just to fit some wheels.
If you do swap to 6 lug parts, AVOID the 60's trucks as that will give you drum brakes and you'll have to have to get your brakes balanced properly, your best bet would be the 90's trucks bake was talking about.
I wouldn't modify the truck to fit some wheels though, it's not a big return on your work.