'86 951: I figure someone else must've had this problem: whatever make/model cables or jumpbox i use, the clamps can't fit onto the red terminal.

I called around and there doesn't seem to b an "adapter" that can fit onto the terminal that clamps can latch onto.

any ideas?

 

My jumper box works perfectly fine on my 86 951 battery terminals.

If yours don't fit, you might be able to modify the clamps on the jumper box.