here's what i would do, from a guy who has six old road bike conversions and two MTB conversions.

1) buy the crank you want.

2) get the rear wheel you want, including the freewheel or track cog.

3) use any old BB that will fit the BB shell and new crank.

4) put it all together and see how far off base you are. then subtract or add, as the case may be, the length from or to the spindle length you used and buy a new one of that dimension.

5) use shims to fine tune. shims can be used, as mentioned, to space the cog or freewheel, the BB, and washers can be used on chainrings. chainrings can sometimes be flipped around. that too can help.

hint. i use a straight edge from the cog/freewheel to the chainring (sans chain) to determine proper chainline (saw it on youtube). works great. i don't have a straight edge, so i use my titanium shafted three iron. don't laugh , it works.