As I recall the 51 Chevy had the upper frame spring mount built into the frame, thus the Must II X-member will not fit.

If you want to go Must II you have to order a X-member that has frame stubs welded on the back of it. The frame is then cut off in front of the firewall.

Fatman fab makes one made like this. The X-member also contains mounting points for the core support.

You might want to consider a 77-87 GM mid size car(Monto Carlo/El Camino/Regat...etc.) or a Camaro sub frame( the camaro is around 3- 3 1/2" more track width.