UPDATE: Was still getting a little push back when closing the action. I had to set the shoulder back .004" for .010" over maximum. I suspect many chambers are like this. I believe the condition with these rifles is overblown...
Believe me, you are in for an Experience in Reloading when working with the .256 Win Mag in the Marlin Model 62 rifle.
As you found the chamber is cut long from rim to shoulder.
The chamber is also cut too large in diameter. Most all lever action rifles have a slight oblong cut at the base of the chamber to facilitate chambering.
The combination of the chamber cut too long with the base too large leaves the handloader in a tough spot. You can use a .357 die to work on the base but then you are unable to bump the shoulder back just a touch to help the rifle lock up in battery.
Not all Thompson Center .256 Win Mag chambers are cut the same. Some TC barrels also give the handloader problems with sizing, tripping the safety and case life.
I quit trying to get anything resembling high velocity from the Marlin Model 62. It has proven to be a reasonable cast bullet rifle with short case life still an issue.
256 Win Mag cases break for me just above the solid head in the pressure ring area. Case strength with high pressure loads is not an issue as you will have short case life with 256 Win Mag cases and 357 Magnum/Maximum cases due to stretching. None of my cases last six reloading cycles at high pressure. There are no issues with primer pockets loosening.
Discussing case life issues with Ed Harris on the Cast Bullet Association forum did not reveal any new tricks with regard to sizing for an over long, over diameter chamber. Ed did suggest making 256 Win Mag rimless cases from 223 cases and this did help some in extending case life. The 223 cases are still worked hard and do not last long. While extraction of the rimless case is not a problem the Marlin extractor will not hold the 223 case securely and will drop the case prior to being kicked out by the ejector.
The Marlin Model 62 will fire with the bolt just short of lock up. I have never seen a picture of this event but I understand firing with the lever not quite locked will break the case and will dump high pressure gas into the magazine.
You can simulate this with a primed case.
The Marlin Model 62 is not unsafe but the handloader needs to be aware of this and be watchful if he decides t experiment with neck sized cases. Just a little familiarity with the rifle gives a good understanding of when the rifles action is fully closed.
Use caution with load data intended for the TC or other single shots as the load data for the 75 grain and heavier bullets is often above the 46,300 CUP limit for the 256 Win Mag.
357 Magnum case rims vary in thickness. This will cause problems as you experiment with neck sizing and setting your dies.
An example of case life: My Marlin Model 62 loaded with 14.5 grains of Alliant 2400 will push the Hornady 60 grain JFN bullet 2,750 fps and I get FOUR loadings after fire forming R-P 357 Magnum cases.
The 75 grain JHP Sierra bullet ahead of 14.0 grains of Alliant 2400 will nudge 2,500 fps with similar short case life.
The 87 grain JSP Hornady bullet ahead of 12.0 grains of Alliant 2400 will deliver 2,075 fps with equally short case life.
Just for fun here is a picture of some broken cases and some 256 rimless cases.
Broken 357 Magnum formed 256WM cases.
256 Win Mag rimless cases