Tough question...they all had thier merits and thier shortcomings. By the way, don't forget that AMC built some street contenders.
Chevy's big block and small block engines were easy to work on and very interchangable. The downside was trying to do distributor work or R&R the starter, especially if just the solonoid needed replacing. Any V-8 between 1956 and 1973 would be a keeper, regardless of cubic inch...
Ford's engines were good overall, especially any Cleveland block. The Windsor blocks were good to a degree and buildable, but the Midland blocks were heavy pigs and prone to dumping thier cranks because of crappy castings.
AMC had three very good engines, the 401 , the 390 and the 304. Downside was that alot of thier parts were from other auto manufacturers that varied by year (pain in the a$$ when trying to get parts for 'em)...
Chrystler did have some good engines, but they also had some real dogs. I think that the 340 was actually thier best all around performer, easy to work on, easy to get parts for and real dependable both stock or built up.
As far as rear ends go, a narrowed 9 inch is the way to go for a decent street rod...if you're going to get medieval, then consider a Dana since they're virtually indestructable.