Sounds like you have a good setup for pig.
A 175 gr bullet at max charge from a normal barrel 26" is 2850 with Win 780, if you used the 175gr Sierra Game King or Berger 180gr VLD Hunting with your longer barrel I'd say you should be able to get above 2900 fps without too much trouble. 2950 with the Berger gets you just shy of a mile before it drops subsonic.
If I had to choose between the 375 RUM and 9.53 Saturn I would choose the RUM. It can launch a 300gr bullet to 2,945 from a 26" barrel (alliant data). Adding five inches to the barrel should keep you above 2,900 fps with a 330 or 350gr bullet from brass that is cheaper and easier to procure (multiple vendors).
The RUM family is all based on the 404 Jeffrey case, so it isn't like it gives up much at all to the 416 Rigby based cases. The reloading data available at Hogdon is a tad anemic compared to the alliant data, but this is one of those cases where you are taking a Safari round and turning it into a long range round, so Quickload might be the best bet for figuring out the optimal powder (from looking at the data H1000 looks like a good bet with a 330 or 350 gr bullet).