Couple things.

First thing, on whether or not the brenneke will work safely...
That's one for you to find out. Think about service pressure limits.
Do you know the operating pressure is of that ammunition?
Do you know what the service pressure is that your drilling was proofed for?
Without knowing this information, even strapping the gun to an old tire, and pulling the trigger with a long piece of string won't tell you if it's safe.
Your gun might survive the experiment, but that might be the very edge of it's limits even if the chokes were straight cylinder. An oily chamber or an especially hot day might turn the leaf for you so to speak.

Next thing - cerrosafe cast your chambers.
Cut one of those brenneke slug cartridges open (or just shoot it in a longer chambered gun) and compare the unrolled length of the hull to the casting of your chamber.
If ammunition operational pressure is within the service load limits of your gun, and when open the hulls stop before the forcing cones, you just might want to consider the 'tie it to a spare tire' test run and see how it goes.

One of our European members might have more 'on the ground' perspective to share with you on the slug-through-choke issue.
I wouldn't try running them in the gun before I had a good understanding of it's intended service load limits, and the mfgr's claimed running pressure of the ammunition.

Good luck.

--Tinker