Will the calipers for the 1991 780 (280mm, single piston large girling) parts
fit on the 1987-1990 - 780 ?

Any issues with calipers hitting the OE 780 15x6 or 15.6.5 inch rims ?

My 90 - 780 is due for front brakes and this would be a good time to
move to the jumbo (large single piston girling) setup.

The brakes will fit the 780 just fine, but I cannot comment on them fitting all 15" wheels. I installed a set on my 760 wagon but for the life of me I can't remember what wheels I had on that thing. I'm pretty darn sure they were 15 inchers though, but what style??? Draco I think?

I have everything here now, calipers, rotors, pads, new lines.
Will test fit the calipers and rim before going forward.