Your old 6x16" locking ring rims 1-pc equivalent is 6x17.5".
The 6-bolt wheels have 7 1/4" centers and are what's called "in-and-out-coined."
When you take them off, do not lose the lug nuts or washers.
Ford and IH also used the same rims and you may be able to find a set of 1-pc rims from a private seller.
Expect to spend and except to wait.
I spent 2K for 1-pc 17.5" wheels after dealing with 2 custom wheel companies and an almost 2-year search.
They are out there, but they will not come to you.