If your installing engine with the low mounted water pump the belt size would be different from Chrislon's installation. What he did was buy an adapter plate that allows you to install the older high mounted water pumps used up through 54 cars and 55 truck engines. I assume you got the shortened water pump that fits in the original 1958 water pump location, which is lower on the block, so you would probably need a short belt. Best way to gauge belt length is to use and old junk belt and put it around the pulleys, cut it where it overlaps and tape it together and take it to your parts dealer. They can size it up for you. You'll want to allow a bit for adjustment in its length to install it.
Alternator? Best is either a Delco (12v) from a car or truck with a built in regulator (saves needing to mount an external regulator) or one of these new one wire alternators can be had in 6 or 12 volt. Newer alternator from the mid 80's up or so are computer controlled so not to easy to use. And many serpentine belt driven alternators rotate the opposite way. Best to match up belt width on all pulleys.

John