Hey Zach. You remind me of a young version of myself.
I was working on a 70 when I was 14 as well, working like a dog, saving up my money, trying to go fast. You will find a bunch of good info and recommendations from some knowledgable members here. I will throw in my $.02 from my experiences. IMO, the right combonation of parts is key. You can throw big money at it, but if everything doesn't jive, you will be dissapointed. For instance, I built a stout SB early on with a rowdy camshaft, single plane, big carb, etc, and didn't have the funds left over for a decent stall and or gears. I put the stock converter back in, left the 2.73's in, and it was a slug. Remember, these cars are VERY heavy, and require a bunch of torque to get moving. As far as your BB build, here's what I would would do. They are getting hard to find, but if you can locate some early closed or semi-open chamber oval port heads(063 castings for instance), you can get a bump in compression. Perhaps sell your 781's or look for a trade. Forget about the single pattern 280 Comp, that is old news. If the budget allows, a hyd roller will be money well spent, but I know of many flat tappets that are still running strong. All of the BB heads have a poor exhaust port that can be "crutched" with some extra duration on that side, there are many threads on the subject. With a combo like this, the bigger valves won't help as much as you think. If the heads need to be completely rebuilt, you might as well do it at that time, but then the chambers need to be opened up to unshroud the bigger valves. Another option is the Edelbrock Hi-Compression heads if the budget is there, you could by them complete and bolt them on. The Performer 2-0 works good, and the Holley should be fine as well. As long as the TH400 is in good working order, it will hold up, just spend some money on the converter. Trust me, it will help get that barge out of the hole. I wouldn't get caught up in HP numbers or E.T.'s, just go for a good thought out combo that you can have fun with and whittle away at it as you go. Oh yeah, I'll end with this. Nitrous Oxide is a beautiful thing, don't be afraid of it. When used correctly and in moderation, it can make up for cubic inches.
Good luck with it!