It is very normal and widly use in our riding life, but how to custom the Ti mountain bike frame for yourself? first you must know the frame size, and the wheel size you will us, also the detail specs options, if you want use the bike as the downhill bike, then you can consider make the head tube as the taper or 44mm, and the dropout for thru axle, these specs options will be high strength and stability.
The very important thing for Titanium Mountain Bike Frame is you need tell us what the crankset you will use, the Q factor and chainline data you will use. and the crankarm length size. we should make sure the crank arm can't attach the frame chain stay. also the fork length size, some rider will use suspension fork for the mountain bike, so we need to know your fork absorb travel size, then the frame will be good match with your fork and crankset.