Joined: Jan 2010
Posts: 9,835
Likes: 1,816
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
MTB's from the early-mid 1980's are going to have short effective top tube lengths.
They typically also have relaxed angles, which won't handle well if you end up needing to use a longer stem, so I would definitely err on buying the larger frame size in this case.
I'm your height and prefer 21" frames from that era.
I would buy an 18" framed modern mtb, but that size from back then would have my knees hitting the handlebar.