I have an old Univega MTB with a triple 28-38-48 crankset and 6-speed freewheel. The bottom bracket is square taper with cup and bearings. I want to remove the front derailleur and convert to 48T crank, keep the freewheel and rear derailler. The final setup will be 1x6. What adjustments do I have to make to the chain line (priority item) and the bottom bracket, if any. The fixed crankset will be 48T so I expect that chain line adjustment should be minimal, if any, on a bike with a rear derailleur.
Assuming the chainline is spot on as it is now, you could remove the inner and middle chainrings, and place your 48t outer ring in the middle ring position. This will most likely require new chainring bolts intended for single ring use (they are shorter). This will only work if the middle and outer ring share the same bolt circle diameter - BCD (most triple cranks work like that, but I've come across a few where the inner and middle rings both had a smaller BCD than the outer ring.)
If above solution doesnt provide the chainline you want, you could replace the bottom bracket. Shimano makes a cheap and (in my experience) reliable bottom bracket for square taper interface: The BB-UN26. It comes in various spindle lengths and for both 68mm and 73mm shells.
You'll probably have to buy a narrow wide chainring.. So that your chain does not fall off. And a longer shaft bb would also be a good idea.