Yes it will work. I just had an M70 375H&H rebarelled to 404 Jeffery. Very little gunsmithing involved. My barrel was a 25" #5 contour PacNor match grade in stainless with a 1/14 twist. All the "smith" had to do really was to open up the barrel channel, make a couple of extractor cuts, put on a different set of sights that I wanted, and install a barrel band for the sling. It's a poor man's 404 but hey it's a 404. Got the whole rig for $2400 CDN (includes price of donor firearm). Got back from the range a couple of days ago and shot a 3 shot 1.13" group using Hornady 400gr DGX bullets. Velocity was around 2200fps.David I am grass green about doing this so it will be a learning experience for me as I go but I have some ideas to explore... This will be months in the making so stay tuned and stay patient
Both cartridges are the same length so it ought to work. However there are some model 70's in 300 Rem Ultra mag which is made using the 404 jeff as the parent cartridge... I actually just bought one (pretty cheap even) to try shooting as a long range model... But I have the option of turning it into a 404 jeff if I don't care for the ultra mag. This way the bolt face should be ready to go and the mag box should be fine as is also?
David get back to me in a week or two and I will try to see how well the 404 cartridges fit and may even have prices on re-boring or re-barreling.
I can put 3 in the mag and "one up the spout".