The simple answer is - yes, you're going to compromise the frame when you cut it. How much is impossible to accurately determine, since a frame of that age could have any number of tiny cracks from manufacturing or corrosion ALSO compromising it - this is why the design process always includes a design factor of safety to beef everything up to make up for these flaws.
I'd say just go ahead and modify the frame the way you want, and just over-engineer the strength you build back into it.
The disclaimer is, of course, that I have no idea what your liability for this job is.
That may not help at all, but that's my take on it.
-Steve
