I had the same idea but I used the HFC M712/C96 model as a base for my build. The HFC and Umarex are almost identical apart from the insides, also the Umarex has a small gap on the base of the barrel which could be a potential weak spot depending how much material you will have to take awayhavew doine had to to fit the bull-barrel. If the Umarex is anything like the HFC i wouldn't worry too much, the alloy is really hard and should be able to take a beating. A few tips that might be helpful...
First of you will want to disassemble the gun (to be able to assess the thickness of the frame and to see if you can thread / fit a locking nut on the inside). Check youtube for a dissassembly tutorial, and watch out for flying springs

I you want to use the gun you will probably have to weld the threaded rods in place. The scope weighs a ton and epoxy won't work. For my build I ordered four new screws with tighter threads, drilled the holes in the frame, then soldered two of the screws to the frame using a threaded rod (through base and screw) as a place holder (the two other screws obviously goes on the outside of the crossbar). This way you wont have to weld / solder the rods to the base of the gun, also you won't mess with the mechanics inside. Like you noticed the mag-well is too wide so you wont be able to fit the grill without modifications... still working on that one, maybe the original plastic grill can be cut to fit? 3D printing a custom one? As for bluing I think it looks authentic and old which is what I like, BC Super Blue and Renaissance Wax gave my gun a great finish. Happy I took Scottjuas advice bluing the darn thing

Some other tips, someone mentioned rubb n buff for the brass detailing on the scope knob. I went with the gold leaf color... looks great! Also have the long mag but 3D printed a plate to fill the hole when used as a display piece. That's about all the advice I can give you without hijacking the thread :lol
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