Jegs crossmembers have a reputation of not always fitting correctly.
All the Gbodies are the same...doesnt matter if Elco, Malibu or Cutlass in terms of crossmember requirements
The only caveat is the later ones for the 200r4, early cars require an extention. But that doesnt affect you at all with a th350
Id suggest either an Iceman tubular unit, or a Gforce (sold as a summit). Both fit excellent and work well.
The iceman is lighter then stock, the Gforce is BEEFY and is much heavier then stock but it helps reinforce the frame somewhat.
Ive run both, I got the Iceman in my turbo car(unit itself is about 7years old), and my old Gforce (10-12years old by now) is still in current use on my Dads elcamino