I don't believe the gas tanks changed at all from 86 onward, although it probably should have as the rear tang is weak as hell and snaps off easily.

Gas tank options are limited outside of stock due to the weird shape of the frame. You can mount just about anything if you are creative enough but it's a give and take. I mounted a Sportster tank to mine by mounting an aluminum u channel with rivits to extend the backbone after the U shape in the frame and mounted the front by drilling through the frame. It works, but rattles a little too much and I'll need to address that due to annoyance but it's been over a year and at least 5,000 miles now and has been going well.

I looked on Partzilla, and what I saw seemed to be suggesting that '86 through '95 was the same geometry, then something changed.

Then I compared pictures from Ebay showing the underside geometry of a ~'87 to a ~'05, and the features looked to be about the same, but with visible slight differences here and there.  It looked to me like maybe the rubber bumpers up front were closer together on the earlier version, and maybe the mounting feature "wheelbase" dimension was shorter, on the earlier tanks, but hard to say looking at a 2D picture.  Some small features looked to have different geometry.

I've had some limited luck, when in this situation, by taking a picture that a vendor has pasted on their Ebay listing, photoshopping in some dimensional arrows and symbols, and asking them to measure and report certain specific dimensions.  The tank on my '06 is dismounted and hanging in my basement.  If you take measurements from your early model, I can take measurements from my later model and we can post both sets here for posterity.   I'd suggest starting with the dimensions I show in the image attached.

L - Length from Center of Rubber Bushing sockets to center of rear mounting holes.

W - Width inside the Rubber Bushing sockets

D - C/C distance of rear mounting holes.

If these three match up, that would strongly suggest you will have a relatively easy time of it, although this doesn't really address "hollow space", that might be required underneath.