1986 Stumpjumper Sizing Question

Hey bike people,

I'm currently on the market to build up a bike for light touring (offroad/paved) and have the opportunity to purchase one of two vintage stumpjumper sports. They seem to be from between 1984 and 1986, where sizing was pretty consistent. That being said, does anyone have advice on what size I should pick if I'm 5'6 with about a 30" PBH. The sizes available to me are 17.5"(listed as 18?) and 19.5". The old catalog has standover heights for the 1988 models but not for previous models. If the 19.5" frame were to have a 30.9" standover height, is that too large? The 17.5 model from 1988 has a 29.9 standover. Also planning to run upright bars. Any ideas welcome, thanks!