The 88mm L/71 that you have posted from RB would have been used on that small turret and I will more than likely use one, but for the E-75 I think they would have put on a 128mm.
I am thinking of using this for the E-75
www.rbmodel.com/picture.php
Thinking logically, the E50/E75 were in their basic forms the Tiger II with modifications. The object was to standardise production as much as possible so they could be built quicker, with bolt on suspension, same engine, transmission etc.
Knowing that the Tiger II was going to be upgraded to a better engine, air conditioning and heating for the crew and a 105mm main gun I don't see why this would not have been done to the E-50 with the E-75 having a bigger main gun such as the 128mm Kanone 43.
This, of course is pure speculation as these tanks were never built, but with the new tanks the Russians were using in 1945 the 88mm L/71 began to have less of an effect and was not the one shot one kill it once was.
It seems reasonable that the Germans would have increased the main gun to the 105mm L/68 originally scheduled to the Tiger II upgrade. It may have been within the project design specs that the E-50 would have two turrets available with the small turret to have the 88mm L/71 installed and a Tiger II turret with the 105mm L/68.
There is a guy in Japan who does this barrel....
www12.ocn.ne.jp/.../sa3528.html
I am tempted to buy a few of them, but they are not cheap at all!
Thoughts and Ideas gratefully received.
James