As per the above posts, the openings in the ejection ports front and rear were to accomodate the longer H&H cases during ejection. I can't remember at the moment if these actions used a different bolt stop for ejection purposes.
As per your project, many early Pre 64's were sent out by Winchester to a 3rd party gunsmith to be rechambered to 300 Weatherby- which is based on the original 300 H&H case. Wonderful cartridge on these actions and works great. I have a 300 H&H AI (which is basically the early version of the Weatherby ) on a pre 64 and it is one of the most accurate rifles I own and puts a 180 out much faster than the 300 WM's I've owned- just can't remember the Chrono specs at the moment. Might be a better avenue to take over a wildcat and brass is generally available just about anywhere for the Weatherby case.
Not sure if the New Haven actions can be modified in the same way. Just not very familiar with the post 64 actions besides my 338 WM Classic stainless, which has turned out to be one of my favorite rifles- now that my son has claimed my 300 H&H AI.
Bob
