Originally Posted by
kaliayev
I have a 90 750 which is the same frame as a 790. It has the same geometry as a 520 with slightly shorter chain stays....
Actually, CS length on your 1990 750 is the same as the 1990 520 and even the mtb frames, specifically
43 cm. The longest CS frames Trek made in 1990 were their least expensve 800-series mtbs, at 43.7cm.
If you compare your frame geometry to the 520, you'll see they're nearly identical.
http://www.vintage-trek.com/TrekBrochures.htm
http://www.vintage-trek.com/TrekBrochure1990.htm
BTW, the OLD spacing on my 1990 950 frame (same as 970, 990) is ~132mm. IIRC, 7-spd MTB freehubs were spaced to 130mm.