First - get yourself a factory service manual for your Mustang... it'll come in handy in more ways that you can imagine.
With it, you can do some research on what parts are located where, their proper function, how they integrate with one another, and how to test them...
The seals under the valve cover are located around the intake and exhaust valves, under the valve springs. Not impossible to change by any means, but it is a nuanced job with some specialized tools (and potential pitfalls).
The (piston) rings are located around the pistons, and they can't be seen without dissembling the engine. However, they can be checked relatively easily with simple tests outside of the engine. A compression test, dry and then wet, will give help you a good idea of their condition. After that, a leak-down test might make sense. This is done through the spark plug holes in the cylinder head.
However, long before I tore into an engine and started changing parts, I'd look for clues. What do the spark plugs look like? Are they oil fouled? 35 years old? Is the PCV valve the correct one? Is the car in proper tune, with proper engine timing? Is the ignition system functioning like it should, with good spark reaching the spark plugs? Etc.etc.
If the spark plugs are fouled (doesn't matter why, yet), then all of the carburetor adjustments in the world won't seem to make a difference.
Once you have details, we'll all be able to help you better. Right now, you'll get a lot of great suggestions but they will all be based on speculation and, worse case scenarios. It might be simple. It might not. But... it is better to approach a problem systematically, and learn along the way.
Edit - Put your location under your name - there could be someone local happy to help, lend tools, bring over beer or, just curse along side you for moral support... :-)