The Whitby is the DeLuxe chassis, and should have better performance than the Super chassis.
Regarding substituting a 21FBP22, the modifications are all mechanical mounting issues and no electrical mods are needed. It is quite possible to remove and save the 21AXP22 mounting hardware and save it for a future time when a good 21AX becomes available, while adding some home-made mounts for the glass tube. There were also mounting adapter kits sold by RCA, but these are rarer than the CTC-5's themselves.
You might find some of this long thread helpful, which is about my work on my Westcott (Super chassis):
http://videokarma.org/showthread.php...I%27ve+blessed
The CTC-7 contrast control change is definitely worth it, as the original CTC-5 circuit tends to run the brightness up and down along with any contrast adjustment. On the other hand, on my set, the contrast is always set at max anyway to get sufficient drive.