Cab and chassis rear frame is 34"
Complete truck rear frame is 37.5"
Cab and chassis rear axle is identical to a SRW axle when measured back plate to back plate, just the spring perches are closer together on the cab and chassis axle.
A true dually axle is 4" wider than a SRW axle when measuring back plate to back plate.
Returning you to your regularly scheduled forum now.