I am thinking about re-barreling either a pre 64 or a more current (2009) Winchester action. I will probably add a new stock too, likely a McMillan.

Would there be any advantage to using one action over the other?

If I buy another stock and it is cut for the 2 piece floor plate, is there any problem with using the adapter to make a 1 piece floor plate fit?

Any suggestions are welcome.

The pre-64 will have the wonderful model 70 trigger, and the action is a little shorter. The 09' will be a long little longer in the action and will handle gas from a blown case/primer much better. The 09' has a box trigger that's not terrible, but not the simple bullet proof trigger found on model 70's up through 2006.

While I love pre-64s, I am down to one. Most of mine M-70s are SS Classics. The reason is it's simpler with a newer action.

The bolt handle has better scope clearance and like mentioned the magazine is a little longer. With a pre-64 if takes some machining to convert a standard length action to 375 H&H length while the Classics and newer actions just need the spacer taken out of the magazine and the bolt stop shortened and the ejector shortened and reshaped--though sometimes there is a little bump at the rear of the magazine well that should be ground down. Sometimes the ejector and bolt stop for a 375 H&H length action will work as is.

It's easier to find stocks for the never M-70s as well. Also, I have seen a lot of pre-64s with sprung extractors and dinged up bolt faces. On the one I have now I had to replace the bolt lock, which meant buying one for another brand or action and then filing it to fit.

I have a 340 Wby on a new M-70 action (started as a 338 Win) and the new MOA trigger is decent--though I don't know why the changed it as the old trigger worked great. Beware the internal magazine well is a little different than the Classic. IIRC, the ejector and/or bolt stop from a 375 H&H length Classic can't just be swapped in, but I was able to modify the existing ejector and bolt stop--and I am just a hobbiest level gunsmith.