From a collector's point of view, yes - most definately. From the shooters' standpoint, doubtful. Any modification would lower collector value, unless the gun is in such poor condition it has no "collector" value. Personally, I pass on modified guns, but that's just my own opinion.
If you are into it for money puposes, not a good move, no.
If it is a shooter you plan on keeping, not a real problem. I'd first make sure the rilfe is deserving of a scope- it should be capable of good groups with a peep, or even open irons from a rest at moderate ranges. Put another way, a scope may not help a rifle that does not shoot well.
Marlin began D&T 39-A receivers around 1955, so any untapped gun is at least 50 years old...
I guess Mr. Joe wasn't clear how old this rifle is. Everybody is assuming he means D&T for a scope. What if he's talking about a newer 39A and wants it tapped for a receiver sight? The subject says "39a value." His actual query just says "39". Would all these opinions be different?
Ahhh...i forgot to mention. I am talking about drilling and tapping for a peep sight. Not on my rifle though (i already had that done). My brother is strongly considering buying a brand new 39A and had some concerns about it, so i figured i would get some other opinions. I personaly dont think it would hurt the value, but then again i really dont care. I dont buy guns to sell them in the future or to collect them, i simply enjoy shooting them and better yet hunting with them. My brother on the other hand...is a colector pure and simple! Sorry for the confusion!
I suggest trying the XS Ghost ring setup. It mounts in the factory scope mount holes. Since it sits on top it doesn't get the way during takedown. Personally I think it looks better and with the post front site, functions better, than the Williams.