Go to anybtruck stop or truck repair/parts place a d they have it. Don't use too much for in the Healey because the radiator and engine are a lot smaller. Another favorite is Bars Leak and it was specified in my handbook when I bought my 12 cup Jag and they called it two satchels of Bars Leak. Going from memory so don't beat me up.
I can also vouch for both Bars Leak and Alumaseal. I had an old car, and little money, during most of high school. I kept my old Ford V8 running on a shoestring, so I used Alumaseal to stop a leaking radiator tank that had a few pinhole leaks at its seam. One application lasted several months. IIRC, Alumaseal was a fine metal powder, and Bars Leak looked like molasses with rabbit pellets mixed in. We used Bars Leak to fix a weeping freeze plug on an F-250, and it worked for a long time, until we finally dismantled the engine for rebuilding, and could then get to the bad plug to fix it properly.
I will add that when I finally had enough money to get the radiator and the tank proprtly fixed, I got an "earful" from the crusty old dude that ran the local radiator shop. The first thing he said when I came to pick it up was "Did you add the GD Alumnaseal to this?" It left a residue, and had also sealed up some of the partially blocked tubes, so he wasn't happy. In the end, after I had asked about and admired the '40 Packard limousine he kept in the shop, he warmed up a bit. He only charged me 15 bucks (7 hours pay!) for the cleaning and repair - probably at least a couple of hours work. He was actually a pretty good guy.
I wouldn't hesitate to try it in an engine that was fairly clean, but if the cooling system is at all gunked up, then maybe not, given the radiator shop guy's reaction...
Note: I just saw Steve's expandable plug, so if I could get to the bad plug, without pulling the head, I would try that first!