I have all the handy Chieftain ref. books, and from looking at some of the pics therein I think I have an answer, but I have been wrong more often than not.
So...I have Tamiya's old kit and want to build a Mk5 with it. I have a bunch of parts from their Chally 1 to upgrade the Chieftain (gun tube, commander's cupola, and gun crutch, etc). But I want to be sure that the following would be a likely fielded combination as the tank was upgraded over the years. I understand alot of the upgrades were added overtime with some overlap (for example the muzzle reference system fitted AND the ranging MG still installed) occured as it was improved.

Here are the option I want to build:
Chieftain Mk5/4C (...I think)
L11A5 gun tube (from the Challenger 1)
Muzzle Reference System on the barrel
Ranging Machine Gun
No 6 Mk 2 NBC pack
APDS stowage box in the left cargo bin
Challanger 1 style Commander's Cupola

I am super dumb on the Chieftain but LOVE this tank! I don't want to spend a tremendous amount of time (or money from Accurate Armour) to build a more common Mk 10 or Mk11 with Stillbrew since I am PRAYING that some day soon someone comes out with a new kit and I am sure it will be one of those two versions.

Thanks,
Dave