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I'm looking to build myself a "cheap" bike that will replace my truck for all but long drives and when something big needs to be moved. I'm self building because quality bicycles are expensive, most new bicycles don't meet my requirements, and there are a ton of old frames that do satisfy most if not all of my requirements.

I'm about 5' 8.5" with a 30-31"-ish inseam. According to most online frame size calculators for a road bike I should be looking in the 52-54cm range and for a MTB I should be in the 17-18" range.
If I want to purchase a vintage mountain bike frame that does NOT have the downward/upward (depending on POV) sloping top tube which size should I use as a guide?
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