I have four different rifles with HS Prec detachable mag setups in them and they all feed very well. I think, per HS Precision, they're supposed to be straight drop in setups with only light sanding possible on some stocks. Mine were all installed when the guns were built so I can't say for sure if they require anything more than light sanding. I'd give HS Prec. a call and see what they have to say. I just talked to them Monday and they've always been very helpful with any and all of my questions.
I know with the extended mag boxes you'll need to have a small little piece milled out to make them fit. You could probably do it with a Dremel but you'd want to have a pretty steady hand if you go that route.