Is this a frame that you have, or are you looking for one?
Many of the vintage MTBs are a little longer than their road bike counterparts. So, you might be able to achieve the tall/short effect you're wanting without compromising the actual fit of the bike too much.
I've done a skinny tire 26" to 700c conversion. But, you might be able to do a mid range tire size (35 or 40mm) 26" to 650b conversion.