Oh My Mickdingo,
If I recall, Winnebago was buying Dodge chassis at that time.
Why does that matter? Because the front and rear wheels may not be the same. This also afflicts some other coaches of the period.
I think you would do well to:
A - Dig around the Winnebago forums
B - Post notes on other forums - Like RV.net Vintage heading
C - Contact wheel suppliers like Southwest Wheel and Allied Wheel - both have great websites.
(if you set up a Skype account, this count be made cost effective)
If you can tell them what chassis, they may know what wheels fit. If not, you will have to measure bolt circle, hub diameter and offset and know what diameter and width rim will work. Do you know what the axle weights are??
BTDT - I have a 73 coach too (not Winnie), first I had to change all 7 rims to radial capable and now I have to replace those with 16x6 since 16.5 tires can be tough to come by.
Be ready for a long hard ride, but be sure you write down everything you learn so you can send it to the next poor fool.
A lifelong waterman and his bride going dry places for as long as the fuel money lasts.