[83Grumman]

I don't think they will fit. Some people have used 245/75/16 (30.47) and they just fit with modification to the rear fender wells.
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[83Grumman]

Really, the rear is an issue? Typically, its the front area that's an issue for larger tires.

Do you have a Promaster? Go out and see how much clearance there is in front and behind the rear tires.