Electrical problems/need help

[406cid70Chevy]

What would cause the headlights to be dim but when the engine is revved they grow brighter? I've had electrical problems with this truck(70 chevy 3/4ton) ever since I had a mechanic put the motor in. I have MSD ignition. External voltage regulator. I have the starter type with the mini solenoid on top of it (piggyback?).

One day I was driving on the freeway and the battery started getting overcharged and began to boil. I replaced the voltage regulator and got a new battery. But since then I've been having problems. Sometimes the truck will fire up no problem, the next time i'll have to crank and crank and crank...sometimes it won't even start. Then other times when I turn the key over all I get is a "click" sound, no cranking at all. Then sometimes I can be driving along fine and all of the sudden the truck dies out. Sometimes I will start back up after cranking for a bit and other times it won't start. I can't figure out what the problem is.

I've also had to replace two alternators that went bad.
One time I did hook up the battery terminals on the wrong posts for a few seconds, but I was having problems before this, this just probably made it worse. Also one I hook the battery terminals on the CORRECT posts I get a pretty good spark...is that normal?

Also what is that little cylinder-like thing right next to the voltage regulator that has a wire which plugs into the voltage regulator?
Maybe that needed replaced when I replaced the voltage regulator.

Well that's the best I can describe my problems. I hope maybe you guys can help me out with the info I've given.

Sorry for so many questions, I'm not the most knowledgable car guy.