If your grandmother didn't beat on it too hard the 400 is probably still in good shape.I'll try to put things in sequence to give you a direction. If I left something out or have the priorities wrong someone will correct me or offer an alternative approach. There are a few more knowledgeable members on CHR.

The first thing to do would be a compression test on each cylinder. If it flunks the test then it'll need a rebuild or maybe a valve job.

If compression is good then you should really pull the oil pan to see what you're working with. Check the bearings and the crank for wear. The bearings should show some wear. The crank shouldn't. If the crank looks good I would replace the bearings even if they look good. They don't cost that much and if you have it apart you might as well freshen it up. With good compression and new bearings the engine can handle a little more horsepower. You could just assume all is well in there but it will cost a lot more to fix later if it's not. When you eliminate the doggyness it's going to put more stress on the bearings.

Next would be a new cam and timing set. After that (or at the same time if you can afford it) you'll need a new intake and carb. I like the "Edelbrock Performer" manifold, carb and cam as they are dyno tuned to work together and you can buy the parts seperatley. It makes it easier for us amateurs.

Other things to upgrade when time and money allow are the ignition system and the exhaust and chrome stuff. And sooner or later a valve job might be needed.