Well, I will agree to be cautious, but disagree on your assessment of the strength of these actions
Martini actions are STRONG.
I have owned, built up and shot dozens of these actions, chambered for everything from 577-450 to 45-70, 303 British, 444 Marlin, 7x65R, 30-30Win, 348 Win, and the list goes on to include 44 magnum and 454 Casull, etc.
I've used both commercial and milsurp Martini actions for these builds and never have found on wanting or weak, which hadn't sustained some sort of physical damage, such as rust, wear from dirt and lack of maintenance or severely crushed/dented from whatever.
I've magnafluxed every one of these rifles I've owned and shot, and only found one receiver which was defective, that was done with a chisel and hammer, from what I could see.
I have always loaded my Martini rifles to modern pressures, with both black powder and smokeless.
Tiriaq has a point about rearward thrust, but the 303 British cartridge has a rather severe taper, which makes that point moot IMHO. It was designed that way for ease of extraction in the field, under harsh conditions and to help eliminate rims ripping off, etc with the cases available during the early days.
OP, even if your Martini is strong enough to handle the Ruger No 1 pressure loads, are you?
Take my word for it, felt recoil from that Martini, with such loads will be substantial to say the least.
I see the rifle has "elevation" hold numbers on the side of the stock.
The previous owner worked those up for that rifle in a manner referred to as "regulating"
He likely also gave you a the load used to consistently shoot to that chart's indication. WHY fix something that isn't broken?
The 45-70 cartridge is no slouch, even at Springfield Trapdoor velocities.
Martini rifles are good handling shooters, reliable and light.
They do suffer from extraction issues when pressures are high. Something to think about.
