Here's a couple of threads you might like to read:

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=95857&an=0&page=0#Post95857

http://www.castboolits.gunloads.com/showthread.php?t=63406&highlight=375%2F303

http://www.castboolits.gunloads.com/showthread.php?t=20340&highlight=375%2F303

It might be worthwhile tracking down some of the posters for more info. I dont have any experience with 303 wildcats but a lot of users that do seem to complain about feeding problems, or can only get them to feed with a certain bullet etc etc.

I load for a 375 Win, 375 H&H and a wildcat 375/356 and all have 12" twists. With all of them, as long as you use the right alloy, lube and the cast bullet fits the throating well, you can achieve speeds of 2000 fps or more with good accuracy.

I sometimes form cases for the 375 Win from 30/30 brass and usually get about 1mm of brass shrinkage.

As far as forming brass is concerned, if the brass is new and unfired I generally just open up the necks. However if the brass is old and/or has been fired before, I anneal the neck and shoulder first and this minimises split necks and/or shoulders. I tend to simply load the formed brass with close to a full load and top it off with a cast pill. This way instead of firing confetti, flour or whatever in the air to fireform the cases, I have useable rounds to use for plinking or hunting.

Regards the throating to use, I think you need to look at whatever jacketed pills you want to use and try to have a shallow throat if possible if you want to use cast bullets (and factory moulds). As well as the bullet Daryl suggested, I'd suggest a 235gr Speer as it should feed well and they perform pretty well.

Cast bullets normally don't do very well in long unsupported throats. Ideally the throat for a cast bullet should be just long and wide enough to accommodate the front driving band.

Hope I havent confused you too much.