>>2023459The old parts are worn with your old cassette, and will need time to wear into you new freewheel, sprocket, etc.
Make sure your chainline is inline and straight. I eyeball it from the rear dropouts to the chainring. On a road double you might want to move the chainring to the "inner" spot depending on your chainline. If you are using a freehub wheel with a bunch of spacers you can easily adjust the chainline at the sprocket via moving of the spacers.
If your chain "droops" it's a little slack but not a big deal, just make sure it's not too tight.
I use whatever chainrings I have for my single speed builds. Most of mine are 110 5bolt so I have an older shimano chainring on 2 bikes. One uses a 39 with a smaller rear cog as a mtb single speed. 130, 135, and 144mm chainrings have minimums of 38, 39, and 42 tooth size so be advised.
My road bike uses a 46-17 ratio which is a little heavy for the climbs but perfect for my overweight size.
I tend to prefer larger rings and rear cogs since they are smoother and wear slower.
I prefer single speed chains like sram pc-1 since they work, are cheap, and I can measure the outside to easily differentiate them from 8speed or more chains. I have heard good things about izumi chains and will try them next.