I'm new to this list...
Have just begun modifying a 20yr old MTB to single speed. Could never stand up on the peddles and get off the seat with the original configuration so decided to pull the bike apart. Have learned a lot in the past month - mainly from Sheldon.
What I've done is remove the smallest sprocket from the freewheeling Shimano thread on and discarded the bigger sprockets. I've then put the smallest sprocket back on but reversed it so that it gives me an extra 5 millimetres toward the centre of the frame. This seems to give the chainline a straight pull from the chainwheel to the rear sprocket. I haven't used a chain tensioner as things just seemed to fall into place. Emboldend by this first success, I have done a similar job on a step through frame for my partner (lady friend).
The dropouts are not vertical but are inclined forward. I've used a big spanner to tighten the axle bolts and that works fine; but I haven't really pushed things. For example I haven't risen from the seat going up a steep hill. (When I was eight years old I dropped on the front bar and haven't forgotten the experience.)
Anybody know of an way of locking the back axle in position?? I'm interested in do it yourself fixes.