Need advice - MTB frame size for stem-riser build
Hi, I'm building up an upright MTB bike for my father, who has a herniated disk in his back. All the upright/MTB setups we've tried in shop have been too low and have aggravated the injury.
So my strategy now is to add a stem riser/extender to a steel hardtail MTB off craigslist - Bridgestone M-series, Trek Antelope, Specialized Rockhopper/Stumpjumper, Giant Iguana, etc.
He's 6ft tall with a 31" inseam. According to guides I'm reading online, his MTB frame range is 18.5-21.5 inches. My question is, given that I'll be adding extra stem length for this upright build, is it better to attempt this set-up on a frame in the smaller range (18-19in), middle (20in) or larger (21-22in). I don't know enough about frame geometry to estimate how the added stem length will effect top tube + stem reach length.
I hope this is enough to go on. Thanks for your thoughts.