I did this conversion on a milled Chinese MAK90. I am not sure how much difference there is in the inside of the Bulgarian receiver as opposed to the Chinese MAK. Dremeling the inside part out was not really a big deal to me. I think the hardest part is laying out the precise location on the receiver to drill the axis hole and locking holes. Themost important part of that, I think, is getting the locking peg holes positioned to where the stock locks precisely when the butt is touching or just about touching the forend. Are you going to put an AK47 folder (with one side locking and dropped angle) or an AKM folder (2 side locking and straight back) on it? I used a Chinese AK47 type folder on mine like the originals. I drilled the axis hole undersized and reamed it to final size so there would be no slop in the hole. I'm really pleased with the way mine came out and the lock-up timing is perfect. It's not something that you can rush through. You need to tripple check your measurements before drilling and cutting.I apologize for the crappy pics.
What he said, I've not built any milled UFs but a couple stamped and your measurements and holes must be very precise, otherwise your stock will either not fold, not lock, or be too loose, depending on which direction you made the error in. If you could get measurements or a template drawn from a completed gun with the exact stock you want to install that would make your job a lot easier.