For a unibody car that is going to get a built engine or extensive floor pan replacement, it is my opinion that sub-frame connectors are a must. Here are some connectors that came from Dearborn Classics. I ended up not using these, because my friend, Jim, and I decided we could build better ones ourselves.
This is what we came up with. We took measurements and used 1.5" x 2.5" tubing that was about 1/8" thick.
I painted them with zero rust paint. Here you can get an idea of what they look like compared to the pre-made connectors.
We then welded them in place and we're ready to move on to the floor pans.