Can I use the battery from my Honda Shadow VT750 on my VT1100
You can indeed use a battery from your Honda Shadow VT750 on your VT1100. It has dimensions of 151mm (6") x 91mm (3.7") x 180mm (7.06"), which is applicable for both the Honda Shadow VT750 and the VT1100.
Depends on the size of the batteries. If the CCA are close to the same and the battery post are on the same side it should work. Note I said should.
Yes, my dads is an 86 shadow 500.