Best offset for 15" rims on front of First generation van

What is the best offset to get for the fronts on a first generation? I'm going to run 15" wheels and don't want any rubbing.

There is more to it than just offset...... 35 years ago, I tried to put 15x7 American 200s wheels with g6015's (2456015 today, I think)
The offset of the rims was to the outside and they rubbed on the front and rear of the fender lips...

My advice today would be to set the axle on jack stands and use a straight edge across the mounting surface of the brake drum to measure the rear clearance to the shock absorber, and then from the mounting surface to the inside of the wheelwell lip at the top....This will give you an idea of how far inside/outside of the mounting surface the tire can be and determine the outer limits of tire width and and inside/outside offset ratio......
It would be my recommendation to take the smaller of the inside/outside numbers, subtract 1/2 to 3/4 inch and then double it for total tire width and go with a 50/50 offset or even inset some to avoid wheelwell rub...
When the wheels are offset outboard, more of the tire/wheel pivots around the outside of the kingpin versus the kingpin being more into the inside of the wheel when the offset is inset so that the wheel pivots at the kingpin or even slightly inside....

On a 15 x 7 wheel a 3-1/2 to 3-3/4 backspace would be common and should allow for a disc brake up grade if you go that route.

I've always been unclear about the word "backspace" because I've seen it defined both ways.... If you mean inset, then it should be good as a front wheel..... 3.5 inch backspace would (of course) be a 50/50 offset of a 7 inch wheel (also good)..... Most of the factory rally wheels of the sixties and seventies were 50/50 offsets.