Originally Posted by mack_turtle

I think you are focusing on numbers in a way that is not really helpful for you. you're over complicating things and making it hard on yourself. you need to let go of that and just find a bike that is made to fit a person of your height, which is not difficult. here's what you need to look for:

• an extra-large frame, maybe XXL if the geometry is conservative. on a mountain bike, this will probably be a 20–22" frame. on a road-oriented bike, this will be a 59–61cm frame.
• to narrow down options, it sounds like you want to get your handlebar rather high. a frame with a relatively high stack measurement will get you there. a relatively tall head tube is a good place to start. these might be "endurance geometry" bikes or touring/ bikepacking- oriented bikes that are designed more for comfort than aerodynamics.
• dial in the fit with saddle position, stem dimensions, and handlebar dimensions. if the frame is designed for someone of your height and riding style, this will not be difficult and should not force you to use a some obscure seatpost or a goofy handlebar / stem.

again, you're starting from a perspective in which you have boxed yourself. you need to liberate yourself from self-imposed obsession with specific numbers.

let's get real- what bikes are you considering and what's your budget? are you interested in buying a relatively modern bike, or scrounging around for dumpster parts and building a bunch of goofy-ass Frankenbikes?

I think you have a strange understanding or reach. a modern medium mountain bike designed for someone around 5'9" has a reach of 425–450mm and an older bike that assumes the rider is using a 90–100mm long stem might have a reach closer to 400mm. you're 6'3" and riding a bike with a 400mm reach would make you look like a bear on a tricycle and would wreak havoc on your spine. are you sure you understand what reach is?