I decided to install a custom made gasket on the joint between the engine and the muffler because I sometimes get leakage through that place even though the screws are real tight. I made the gasket from the leftover gasket material of a Ford Aspen carburetor. I think they make those from asbestos and lead. Is this a good idea?

I dunno, but I once made an front seal gasket for my p/u out of an old felt hat..

I make gaskets from thick card stock. The last time- I installed a muffler extention, and used the top of the package it came in- hard card stock....worked great!

In my opinion the best gasket for that application is Silicone Automotive Adhesive Sealant. Comes in a tube. I have been using it for about 10 years. Trabaja perfecto.

I have been using the High-Temp RTV Silicone gasket maker from Permatex which also works fine and there is no need to cut a card.

If the two mating surfaces are straight, smooth and made of similar material, you often don't need a gasket at all. Gaskets tend to blow put causing leaks, loose mufflers and poor running engines. A thin film of RTV silicone doesn't hurt though and can help with oil seepage. If your application can't be sealed without a gasket though, it sounds like the one you made should be fine.