Is it a VTX1300T, R or C? First two have 17" front tires whereas the C has a 19" front, right?
VT750 has a 17" front tire.
VTX1300 T or R has a wider stock tire (112MM versus 108MM) but I don't know if that's enough difference to matter. Have to check clearance. The axle would be my biggest concern.
1300 tire is also taller by 1.575" but that won't make much difference if there's enough fender clearance. You're talking testing so I would expect enough caution to prevent full compression of the forks.
What are you testing? If it's uneven front tire wear/cupping on the 1300 then save your time -- that is an oft-noted issue on VTX1300s. AFAIK, answer/solution is yet to be found.