Were these actions the same dimension on the outside? Where were the differences made that allowed the standard action to feed the much longer H&H cases?

If you are referrring to the pre-64's, they are all the same length. The difference basically lies in the loading port opening and the mag box, longer in the 375 by about 3/16" or so. The standard 243 length has a shorter box and bolt stop on the bolt. Other things are there also but the basic lengrh from receiver face to rear tang, all the same length.

Which always amused me - same action for .22 Hornet as .375

Cuts down on overhead that way.

I'm just wondering if you could open up a Pre-64 standard bolt face action to the longer feed ramp and magazine box to allow longer cartridge overall length?

All pre-64 actions are the same length. The various calibers were accomodated by longer bolt stops and blocked magazines for the shorter cartridges, and shorter bolt stops, longer magazines and corresponding magazine wells in for the H&H Magnums.

The H&H actions also have a slight indentation in the receiver ring and a modified bridge to allow ease of loading for the longer cartridges. This modification does not weaken the action and it is not difficult to do with the proper equipment, using an original Magnum action as a pattern.