Dixie Dude,
I think you are very likely correct about the Mauser action/extractor being responsible for your success.
I am assuming you are also opening the case to .35 and seating a 35cal bullet, plus having some amount of false shoulder.
I had a 30 Gibbs, in which not only is the standered 30/06 case taper mimimized, but the shoulder is blown waaaaay forward.
Some folk and as I recall, Gibbs himself did fireform standard 06 ammo to 30 Gibbs, without case forming or long seating bullets.
This must have been made possible by use of the extractor to hold the case in position as a typical "push feed" action or in Doug's case a #1 action, would simply push or allow the 06 case into the larger chamber and contact between the firing pin and primer would be unlikely at best.
Personaly, I didn't like the sounds of the case being held in position (in forming such as 30/06 to 30 Gibbs) by the extractor, so always chose the long bullet method.
If I had started with 35 Whelen brass I could have put a false shoulder in place, allowing a bit of a crush fit as the action was closed and the forming process would have been quick and simple.
Maybe we could all look for a source of NEW 40 Whelen brass
(fat chance of that!!)
and help Doug solve the problem.
Keep em coming!
CDOC