Phil - a couple things that if you are a total 'newbie' to street rodding and the mix and match to make things work.

Boxed frames will not be from end to end. Most builders will leave them open from the front cross member forward and the rear cross member back.

An 8" differential will work fine for street use and modest horsepower, i.e. 350+/-. They can be difficult to tell apart under a car, but the 8" usually has a couple dimples in the back cover and that cover is always rounded while only a couple of years for original 9" are that way ('57 and '58). There is one good source of differentials in Michigan, John's Industries. Ring and Pinion Gears. A friend has one from there in a '32 and it was nicely done. They can do the brackets and shock mounts, but you have to tell him what kind of attaching bars you will be using - 4 bar or triangulated.

Again, tear it completely apart, find out where you are, know what you already have then do your basic build plan. I'll preach that until you get sick of hearing about it