Either cartridge will do well...I built a x62 on a mauser action because the Brennele was too long and fat. For a W70 I would pick the Brenneke because you can load longer...maybe a bit of work on the rails and bolt face, I'm not sure...no extra work on the x62.
I would go with a new barrel rather than a rebore as the cost should be about the same. You could also do any number of "wildcats" with a new barrel if you are a reloader...a 375 or 416 "Brenneke" would rival the 375 or 416 Taylor and give you 400 gr plus bullet weight in the 416 if you want and have plenty of whump for any game in the world.
I'm getting 2350 f/s with 300 gr and 2400 plus with 286 gr without pushing the envelope.
Lots of x62 ammo availible and cheaper than Brenneke stuff, but the Bren HAS a bit more thump on both ends...2400 vs 2700 with a 286 gr bullet...something to consider. The exta whump on both ends isn't needed for N.A. game but for the rest of the world I would go the larger Bren and a 416 cal.
Pick one for the conditions and be happy.
