Rear cab mount crossmember help

[mechanick]

I ordered the two (rear cab) crossmember pieces to replace my rotten ones. However they aren't going to be long enough once I cut the rot out. Does anyone have or do they sell the entire thing. I'm talking about the one in the rear of the cab floor that looks like c channel and has a hump in the middle for the tunnel- driveshaft. Welds half on the b pillar and inner rocker.... What do I do???