Your spot on with the gear boxes that is what they are. the 5 speed looks like the 1200 gearbox gear shift, the 4 has a rubber insert in the shift feels a bit of a dogs breakfast to change I have fitted the 4 speed but not to late to change to the 5 if that is the better box .

The 1600 trans had that weird monkey motion linkage at the rear but your 5 speed, presumably from a 200B SSS will have the newer & much improved arangement.

It might take a little more effort to fit the 5 speed, but in the end, I think you're going to need that overdrive ratio.

Whats it going into?

edit. I just remembered the L18 conversion for the 1200.

Go the dogleg. It is easier to change the box later with reguards to different options. You have already sorted the tunnell out which is half the battle.

I have a 4 speed from a 1600 and a dog leg long 5 speed. What is going to be best ratio behind the L18 with a 3.889 diff .At the end of the build ,it will only be a sunday car, unsure wich box to fit