Other guys will be able to help you a lot more with the engine work. My suggestions lie with the tank and carbs. After 21+ years of sedentary life, whatever gasoline was in that bike is now solid varnish.
When your engine work looks to be near completion, start tearing into your tank and carbs. Don't make the mistake of cleaning your carbs thoroughly and then running a dirty tank. It'll clog your carbs quite quickly. There are a myriad of ways to clean tanks explored here. I'd be a little worried, maybe, about rust pinholes and things if it's sat that long, but the only way to know is to try it out. (well, that and look for little bubbles in the paint) If you're good with following directions and don't mind playing with electricity, electrolysis may be for you, as it may spare your brilliant orange paint. If not, or if the paint's pretty far gone, you can try The Works drain cleaner but be careful to use enough baking soda and water to neutralize all that acid afterward, or you'll get a bad flash rust. Directions are all over the forum somewhere...
Invest in Berryman's carb cleaner or Yamaha Carb Clean (yamaha has my vote) and completely disassemble. I don't think the '73 750s have the amount of rubber bits and o-rings the 650s have, but you'll still want to remove them all before your carb soaking.
Invest also in new OEM carb kits. They're not too pricey and will give you peace of mind. Take out all your jets and clean them with twisted copper wire so you don't alter the soft brass jets, but get the crappy calcium/bromide/varnish sediment out of them. Your carbs will probably come off four or five times before they stop acting like they're clogged. Eventually you'll get good at removing and installing them.
I probably have more advice rattling around in my head that'll come out eventually. I was 21 when I bought my bike... now 24 for my first summer riding it. Your project may progress more quickly. Best of luck.
