Hello Russell,
You say your proposed adaptor willl lift the Cross slide, I presume you mean the new Vertical slide?
If this is the case, don’t worry about it. If Andy is right and you have a slide as he describes; go for the adaptor plate idea and preserve the original. The bonus is if you replicate how it was locked onto the original cross slide, you have the advantage of being able to swivel it.
If on the other hand it is just a plain boss, carefully cut it off and keep it somewhere safe. If you then need to revert, just clean up the mating faces and screw it back on.
I was given a Myford Fixed steady which had a square boss to locate between the bed shears of the Myford. To use it on a Prazimat it needed raising to centre height, so the boss was cut off to fit an adaptor plate and a new clamp plate.
This has been a total success in use, but should I wish to pass it on to a Myford user, then with the boss restored it would be almost as good as an original.
I hope this is of some help.
Regards
Jim