IME, old road cranks, don't work well on MTB frames. the geometry of the crank is wrong. the arm is not offset enough from the spindle attachment point and so necessitates BB spindles lengths that are excessive. i've tried it... and failed. but, if you can find a BB spindle that is long enough, (i couldn't, but admittedly didn't look very hard), it should work. there are ways to add a couple millimeters clearance. smaller chainring, spacers on driveside of BB or inside of crank bolts, but they don't add much. take a look at the chainstays on the MTB frame and compare them to a road bikes's chainstays. they sit out much, much further in order to clear wider tires.

i just ended up putting my nice old road crank back on the shelf and lived with the beatup looking MTB crank the i had.