First off, Welcome to the board!
Second, you covered alot of ground in your first post, so I'll try to give advice in a condensed form.
Suspension: There's alot of options out there right now, but be sure to shop around and read real reviews and real world experience before jumping into something with the best price. Keep in mind there aren't many good option to improve geometry and handling that still utilize the stock crossmember. The Ridetech StreetGrip system would be about the only real option for keeping the stock crossmember. The next options would be a whole new bolt in suspension. There's only two options I'll recommend for improved handling and ride. 1)
https://totalcostinvolved.com/produc...evy-c10-truck/ I have this in my 68, and I'm willing to answer any questions if you're interested. 2)
http://nolimit.net/products#!/~/prod...32&id=26810473 Rob builds a nice product and stands behind it. I personally don't have his products, but I compete against a few that do. I'll only recommend these two because I've seen them in action and THEY WORK!! Yes, this all may seem way more expensive than some options, but I've done lots of research and I've learned that you get what you pay for. Stay away from Heidt's and CPP stuff.
Engine: I really don't need to say too much here, since you seem to have what you want sorted out. I do want to mention that an LS3 525 crate engine will have all the power you're after and will make that power consistently with great street manners. I know quite a few people with 500+hp 383 and 400s, and my 530hp LS1 has given me no fuss about idling and running in any condition while they're overheating and loading up and fouling plugs. Just my .02
Rearend: I've been beating and autocrossing on a 12 bolt with 500+ hp for a few years now with no issues. The only upgrade has been an Eaton LSD carrier. Yes, I'm still using stock axles and a used 4.11 R&P gearset. I may upgrade to Moser hubs and axles this winter, but my stock c-clip axles have held up well. Even if they break next event, I still can't complain. If you're just wanting a hefty street bruiser and not an all-out race truck, then going 9" may be overkill. However, a 9" is always a good investment, and has tons of aftermarket support.