So you won't be confused by "case head size"... its the internal diameter of the case in square inches, not the external size that is used to calculate back pressures.
Both the 307 Win and 356 Win are the parent cases used for the Super Bower wildcats that were developed for the Contenders (22, 6mm, 6.5mm, 7mm, 30 Alaskan, 358 & 429). So improving the parent cases can allow the wildcats based on otherwise no-no cases to be shot safely on the Contender platform sometimes.
So if you want a 30 caliber with a little more oomph for your Contender, the Bower 30 Alaskan, SSK 309JDJ or 30-30AI will all get it for you safely.
L.
Edit to add: The Contender was designed to operate at up to 48,000 CUP.