Ben:
In Australia we pretty-much subscribe to the philosophy in Graeme Wright's book, that if you match published 'factory' velocity from the period with any modern medium or medium/slow powder then you will be operating at much lower peak pressure than the original, and should be quite safe.

Its not that we religiously follow Graeme's book, rather Graeme's book is the compilation of everyone's experiences and pressure-tested loads 'down-under'.

"Can the old guns use loads such as Marrakai's Greener conversion??"
Absolutely!! But as Curl says, work up from below for your own peace of mind. The chronograph is the single most-important piece of reloading equipment for the NE-reloader by far! Do not exceed published velocities of the day with any given bullet weight.

BTW Curl, I would be interested to know which N-for-B cartridge reloads dislike Varget in your experience. I have only found it wanting when barrel-lengths are shorter than 24 inches, and then only because the muzzle-flash is a fire-hazard when hunting!