1966 Lemans..Wanting to put as large a rear wheel and radial tire on the back without cutting fender/ wheel wells or relocating frame...what size 15 in -wheel and tire  do I need and what offset do I need on the wheel?

The max wheel size will be either a 7 or 8 inch wheel with either a 4 or 4.5 inch back space. Factory wheels were typically 6 inches.

Now if you want absolutes, invest in an inexpensive Percy WheelRite Wheel and Tire tool from Summit Racing. It will let you measure backspacing, offset, and give you the max tire size you can fit within your wheel wells.

https://www.summitracing.com/parts/php-01201