An interesting Idea.
I'll proffer a different approach .Rather than just look for a turret to plonk on a mk C...Start with the Whippet and an Austin.
Why?
Because it's more economical and there's no waste.As you'll end up with two unique vehicles rather than one and bit for the lead pile.
This is musing is one you don't need to have much skill for and will take very little time to achive.Plus you'll not have to yry and match detailing such as rivets . As it preserves the existing details of both kits.
The Whippet comes with separate tracks so it's simply a case of reversing the body between them.
To change the body simply cut the crew cab away form the engine compartment. This is done with one vertically cut where the cab meets the angled engine hatches. Then one horizontal cut below the horizontal rivet panel which runs from the rear of the body and under the crew cab.
Once separated keep the crew cab ( don't bin it.)
The Austin body requires one more stage but in principle its very similar.
First stage is to remove the rear mud guards. This is done by repeatedly scouring the connecting point to the body from the underside in the wheel arch.( works best and fastest with the back of a blade tip not the cutting edge.)
Once released from the body put them to one side( don't bin)
The make a vertical cut directly behind the drivers cab.Then a horizontal cut flush to the underside of the curved section, and toward the rear of the vehicle.
Once done you should have a secton with the turret area's and rear cab complete with the two storage boxes still attached.
Your tank is then simply a case of thaking the Austin turret body glueing into the space left by the original Whippet crew cab.Then added the reversed track so that the storage boxes are over the tops of the highest point of the track units. Then add the Austin Turrets point to what would have been the rear of the Austin body.( now the front of the tank.)
You may choose to add a glancing plate below the lower leading edge at the front to camoflague the join.( a bit of mud will eliminate the need to add rivets.)
An novel looking armoured car can then be made from the left over Austin Body and the Whippet crew cab.
It will have to be raised slighty. But this can be camouflaged with a storage box either side.The remaining flat bed area can be covered with a couple of planks .The re attatch the Austins rear mud guards.
What you'll end up with is something similar to the Single turreted Austin in the bottom picture.
If you it's an fancy doing then feel free to pick my brains As I've more than a passing familiarity with these vehicles

I've included a Laffy- White as well just to show it wouldn't work quite as well but it is still possible.
Mark.