Hi, I did this swap a couple years ago into my '55 chevy 2 door sedan. I did not install a crossmember and, to date, no problems. I would not go to the trouble.

Chad

1955 Chevrolet 210 2 door Sedan
a 30,000 mile UNMOLESTED original, now my daily driver! Original 6 cylinder with a rebuilt '66-'69 Saginaw 3-speed Overdrive. Synchro low! Otherwise, everything original down to the spare tire. Interior is like new. Needs paint, but absolutely zero rust.