Can someone guide me on what type of solder nitro exhaust manifolds typically use? Also need to know the proper flux. Trying to do a custom thing here. Thanks

First of all you need special solder for aluminum.

You have 2 simple choices.
1) a so called solder stick
2) a aluminum solder starts kit

Both can be found on Ebay etc.

I picked up some Bernzomatic AL3 sticks, I suppose I'll give that a try. Whatever the factory used, it melts near the temp of the base metal. By the time the solder melts, the aluminum parts are like play dough.

You need the correct stick like Roelof mentioned. Also need to control the heat and get on and off the part quickly. MAP gas for a DIY torch or an Oxy-Acetylene setup are best.

That's the thing, I need to figure out the proper stick. I'm good soldering and brazing steel, brass, etc but aluminum is a pain in the butt. Tried the AL3 sticks this morning, can't get it to flow into the joint at all. I just need to figure out what the factory uses.

I TIG welded a few, but I have been TIG welding for over 30 years. Maybe you can go to a weld shop for help.

TIG is definitely an option. Maybe this is a good excuse to get another tank of argon.. I've been putting it off for a while.