Yeah, I've use it. It's tough stuff and has to be 750 F degrees to melt, way under the melting point of aluminum.
Is there evidence of torching?(AKA tunneling). This is when the hot gases cut grooves in the aluminum then the Alumiweld would work. Totally degrease the jug then pre-heat in an oven. When I used it a small propane torch wasn't enough heat. The aluminum carries off heat so fast a small torch couldn't keep up. I used a torch hooked up to a 20 lb propane bottle like on a BBQ. Maybe with pre-heating in an oven at the highest temp you could use a smaller torch.
To level out the top of the jug there's a thing called draw filing. A file can cut very finely if you hold it sideways with both hands and pull the file towards you using the file teeth on a sharp oblique instead of the more direct way of pushing it across which would cause gouges. Holding the tang or handle in your left hand and the tip with your right pull the file across the top evenly. If you have to file more on the Alumiweld buildup. Try to keep any buildup to the least amount possible so you don't have so much to take off. When you have it milled to the right level the file will almost float across the diameter of the bore. When you reach the point of where it seems to not be working anymore then switch to the sand paper and glass routine. Be careful not to take very much off the top of the cylinder if you can help it. If you do you can still use two base gaskets, that would just change the port timing slightly.
Spray the head gasket with Permatex Copper Spray Form-A-Gasket and let it set up like it says, torque to your favorite torque setting and you should be good to go.
p.s.: I don't think welding the head to the cylinder would work. I don't think you could depend on the Alumiweld which is more of a solder really. You would have to depend on a capillary action of the solder to make it to the cylinder/cylinder head junction inside the cylinder. Fail at that then you'd still be getting oil down the stud holes