Last I heard, there was no such thing as radiator sealant in a commanche. If you have a hole, you need a new radiator. Try tightning down all the hoses first, though.
an nv 3550 transmission would be good.. an nv 4500 would even be better but you would have to buy an adapter
This sounds like an earth problem check battery to chassis & chassis to engine earth connections also check earth to alternator best wishes
4.5 quarts depends on what 6 cylinder. The 4.0L takes 6 quarts, the 3.7L holds almost 5 quarts.