It occurs to me that there are some "fixed points" that can be used to assist in deciding if the Whispertones will fit.
The two "fixed points" are the exhaust ports and the subframe mounts for the mufflers.
I would measure the distance from the mount to the point where the mufflers mount on the header pipes on the mufflers you have, I would also try to get a sense of the "proportion" of muffler on either side of the mount point - then compare to the best photos you can find of the Whispertones.
Next measure distance to the end of the headers on you existing pipes, also, look generally at the headers on the bike, do the ends line up or nearly line up with anything reasonably "fixed" like the foot peg or the gear shifter etc.
Then look at the whisper tone mufflers, if they seem to project forward from the subframe mount at least as far as the ones you have, then they can be made to fit - if need be you can shorten up the headers. But if they do not project as far forward you may have more complex problems.
I would not worry too much about the 36/38mm problem, a strip of 1mm shim stock can be easily cut and used to ensure a leak free joint.
