Hi Michael-

That’s a very general question, so I’ll try to provide a very general answer…   The premium prices paid by collectors for pre-64 Model 70s are for rifles that are 100% correct and in high condition.  I said “100% correct” because I’m hesitant to use the words “original” and “pre-64 Model 70” in the same sentence…

Any “permanent” alteration to the rifle, i.e. something that cannot be reversed by simple replacement of parts, takes the gun out of the “collector” category.  It’s value is then driven by what it’s worth to someone who wants a rifle for hunting, shooting, or even its component parts…

To use your examples, IMHO a reblue or non-factory recoil pad will reduce the value by about 50%.  OTOH… Missing sights or front hood only reduces the value by the cost of period correct replacements.  A reblue, is probably the worst, since it cannot be undone.  The rifle will appeal only to someone who wants to use it…

A shortened stock, in theory, can be replaced, and if the replacement is period correct and happens to fit the gun, it can often be (nearly) undetectable.  Whether that’s a worthwhile endeavor depends on the gun, it’s condition, chambering, style, period of manufacture, etc.  A high condition original finish pre-64 Model 70 stock can easily cost $500 – $1000 depending on vintage and style.  So putting a $500 stock on an $800 mid-1950s 30-06 Standard rifle in so-so condition makes no sense.  It might make the rifle worth $1200, but it’s not enough to recover the cost of renovation.  

As for sights and hoods, again depends on condition, chambering, style and period of manufacture.  Getting a Lyman 16B and correct hood for a mid-1950s Monte Carlo comb standard rifle might cost only $100, hence their absence devalues the rifle only by that amount.  But a Lyman 48 receiver sight for a pre-war rifle could take a while to find and cost several hundred…  Usually not worth it for a “shooter” but it might be if the rifle is an otherwise correct high condition gun simply missing those parts.

Just my take…

Lou