Just got a truck. Now I have some questions

[KD4UPL]

Hello,
I just recently purchased a 1969 C10 shortbed. It had been in my Grandfather's garage for at least 20 years but he never drove it.
I've been researching all the VIN codes on the truck. It came from the factory with a 6 cylinder. It currently has a 1971 307 V8. The engine was originally attached to a powerglide but it currently has a THM-400. The rear axle is a GM 12 bolt from 1970 that came with 2.73 gears and an open diff. It now has 3.23 gears and a posi. The rear suspension is leaf springs.

So, my questions are:
Is the leaf spring rear correct? I thought most of these trucks were coils in the rear.
The trucks needs an exhaust system badly, I'd look to put on headers. What in the world do I order to fit the engine and clear the frame and other parts of the truck?
The charging system is putting out 15.1 volts at the battery according to my volt meter. I guess I need a voltage regulator or should I just change to a one wire alternator.
The side marker reflectors (lights?) on the front fenders have no bulb and I don't see any wires or socket for one. They look like they have a spot on the back for a bulb. Is there supposed to be one there?

Thanks very much for any help you can give. I'm sure I'll have more questions.