Needs repeating...

A 53 cm road frame and a vintage 19 inch mountain bike frame are going to have very similar top tube lengths if they are both built with traditional geometry... the only difference between them is the tire size used as these were basically touring frames designed for a 26 inch wheel.

As such, they take well to being converted into touring and commuting bikes.

If the stand over and top tube measurements are comparable the bike may be an excellent fit.

My hardtail is a 20 inch frame with modern sloping geometry... by my specs I should be riding an 18 inch mtb but find the top tube length leaves me feeling cramped and I like to stretch out a little more.

The Op needs to clarify what he is riding and what his potential mtb is as then we could make a better comparison between the two bicycles as there can be a lot of variance between different models.

My Kuwies are 55cm square and essentially share the same geometry as a modern Surly LHT which is really a retro design and not anything new.