I would suspect that if you have not have a tire wear problem with current set up as long as you use the original mounting holes you should be good to go. The string from the center of hitch to axle is a tried and true method which is what I was told when I got my 8000 torsion bar axle for my heavy duty utility trailer in 2002. It could not be asked to tow any truer.
I am in the process of upgrading to 5200 lb axles and the factory bolt holes are correctly spaced for the new axle. All I will have to do is open the axle tube opening in the side plate and I should be good to go.
You will probably find there is a goodly amount of movement in your bolt holes which will allow you to move axle a bit.
My new axles have elongated bolt holes in the mounting brackets which will cover perhaps up to 1/2" inch of movement.
The center to center hole location on my Sovereign
frame is around 9.050". I have gages fabricated that go from 9.040 to 9.070 C-C and I would not be surprised if a 9.080 would not align due to the hole size differences on the high end and 9.030" on the inside dim. FWIW my holes run .035" to .039" oversize of the bolt diameter.