I too have considered rechambering my .35 Marlin 336 to either the .356 Rimless (.358wcf brass loaded to .356wcf specs) or outright conversion to a .356wcf. To convert the rimless .35 to .356 rimmed, the easisest way is to get a .30/30 bolt which is set up to handle the rimmed case.
Either way, after doing some load development and chronographing, I've got a load in .35-Rem. with the 180gr Speer that runs 2,500fps and extracts effortlessly, and a 200gr Rem.Cor-lokt thats running 2300-2350fps. I can't now justify the modifications, as the improvement in power and trajectory dosen't justify the cost.
We really should be clammering with Marlin to start a "Custom" or "Classic" series with a different caliber offering each year; some in "New" cartridges such as the .356wcf, or a 7-30 Waters (I want one in each !), others in oldies such as a .25-35 (want one of these too).
Especially, if anyone is listening; is a 20" half magazine Stainless Steel-with gray finish (like some of the Ruger SS guns), with either a synthetic stock (camo) or laminated, and no blast ports please!! in .356wcf. Also a 7-30Waters too !!