I wouldn't alter a nice Russian either. It will most likely always have some additional value to collectors as a genuine historical artifact. With so many other choices out there which wouldn't depreciate nearly as much with the modification it would be something of a waste, IMO.
FWIW, some SKS receivers are very hard. Drilling and tapping this sort of material successfully can be a somewhat delicate operation, and it is all-too-easy to inadvertently snap-off a drill or tap in the process. Removing the broken tool is a real PITA, and the results of an unsuccessful attempt can be irreparable.
The results of my own experience with a receiver cover mount have been quite positive. The job must be approached and performed with a great deal of care and patience to obtain good results, but it isn't technically difficult.
The crucial factors are maintaining the exact angle of engagement on the forward "points" of the mount when removing material and removing only the barest minimum necessary to obtain a slight, but firm, resistance to closing the take-down lever pin. This is most easily accomplished by the use of a clean, high quality file, Prussian blue or other layout fluid on the areas where material is to be removed, and a firm tap with a non-marring rubber or plastic mallet applied to the rear surface of the cover with every fit test to determine exactly the degree of contact and where it is being made.
The Keng's mount I fitted to my commercial Chinese model with this method has virtually no discernable vertical or lateral 'play' after at least 3K rds. and returns to original 'zero' to within 1 'click' of W or E (at most) after dismounting for cleaning and reinstallation. More often than not, no adjustment is required. Plenty good enough for my modest needs, and on the same order as most of the high-buck "QD" systems I've encountered.
