Which model 70? The post '64 M-70's were all magnum length rifles and are perfectly capable of achieving the full length magnum rounds with minor modifications (i.e. new bolt stop, new ejector, remove filler block from magazine box). The M-70 has actually been offered in the .300 Weatherby, which is just a 340 Weatherby necked down to .30 caliber. Easiest way to get a .340 is to buy a .338 Winchester, alter it for full length magazine and bolt stroke and have it rechambered for the .340 Weatherby. I have done this. I modified my .338 win for full magnum length bolt stroke and magazine and had the chamber "long throated" so that I could seat the 250 Grain Nosler partition bullet with the base flush with the case neck. It worked too. I get over 2800 fps with that bullet. I did not go all the way with rechambering to .340 Weatherby because the .338 so loaded was all the punishment I could endure and it was within 100 fps of the .340 Weatherby anyway. Why bother?
The short answer to your quesion is yes. The M-70 is capable of being made into a .340 Weatherby both safely and economically.
