Take it from me mate, the CEG is quite simply the CL's 8P with some minor functional mods - the ratios are the same. I have turned the mofuggin shafts by hand and marked 'em off with chalk just to be absolutely sure! (I wanted to make sure that, after sweating blood to change the bloody things over, i wasn't going to be left with an automotive turkey). Those numbers are again (to the best of my recollection, and assuming they're both on the 4.0625 diff with stock tyres)...
CL ------------------------ GT
mph/1000 - ratio ------- mph/1000 - ratio
4.5 --------- 3.454 ------- 4.5 -------- 3.454
8.0 --------- 1.958 ------- 7.5 -------- 2.087
12.5 -------- 1.250 ------- 10.8 ------ 1.447
17.6 -------- 0.891 ------- 14.4 ------ 1.089
21.2 -------- 0.740 ------- 18.5 ------ 0.851
as you see, there's not much in it between GT 5th and your 4th
(there's also the little seen hardcore edition with 4th=1.122 and 5th=0.891)
In standard 74hp guise they'd probably pull about the same speed (one in 4th, the other in 5th), but point a pair of 88'ers like yours down the bahn and you may find yourself briefly left behind, but then pulling away quite nicely through the top of 4th and into 5th.
My money's firmly on that - 8P/CEG boxes are common as muck and the flies upon it, very widely used through VW's 085-utilising model ranges. There have previously been examples of broken-GT owners appealing for replacement boxes as they can't track any down and not getting joy for ages - perhaps a previous owner decided to cut their losses and just get the car mobile again. (The ultimate GT box would be the stock one with the 8P 5th bolted on as 6th... too bad there's barely room in the engine bay for the standard model!)
Know what you mean with getting peak power at motorway speeds tho
the ashtray sits precisely at peak torque around 65, it's great for powering up hills & leaving traffic behind, without going near the stick, when the slowarse in front finally pulls back into the middle lane... (its also very nice getting peak in 3rd about the same speed, when on a charge and doing a bit of dodgery)