Another good option for MTBs is a trekking bar. That way your investment in parts is low since all the original parts will work. Trekking bars work well with longish top tubes since you are not quite so bent over and they have lots of comfortable hand positions. Also some of the older vintage MTBs from the 80s tend to have a bit longer wheelbases and the top tubes tend not to be as long relative to seatpost length than the ones in the 90s. They were still working out MTB geometry in the 80s and the bikes were often a bit longer in the wheelbase than became common by the 90s. My 1988 specialized stumpjumper comp came with a u-brake in the rear. It's a 20 inch bike with a 22 inch top tube; my 1991 team specialized stumpjumper is an 18 inch frame with a 22 inch top tube. Big difference when thinking about adding drops.