RN used an early UK spec tub on that pick-up conversion. I have some tubs with the rear bulkhead and I'm not sure, looking at them, how you would get any Rover "b pillar" parts to work. It would need to be fabbed.
The roll cage from the ECR truck is in an HCPU which is somewhat different. Also wasn't actually mounted to the frame, just sitting in the tub and on the fenderwells. I would suggest frame mounting if it is intended for anything other than looks.
Off topic; As for the white truck with the dual tanks. I don't know who installed it but the saddle tank is a standard UK / Euro spec using the corresponding filler, in addition to the NAS spec tank and filler. Uses a transfer pump.
Side note; The late Euro spec tubs are available with a rear fill and the rear bulkhead for those that would desire a PU with a "NAS correct" rear fill.
new expansion complete. Not only are we the only Rover shop in Eliot Maine...now we're also the biggest.
"Dedicated to the resurrection of junk through engineering?"