I'm quite fond of mountain molds, they are custom made to your design, and the cost in't much more than some of the production molds.

For the 416 mold, go with the RCBS, it seems to be a good shooter for those that use it. You use 41 cal gas checks, should be able to pick up the mold and checks from any of the mail order houses.

The 50 cal mold will have to be either a custom, or from one of the smaller shops. If cost isn't an issue, look at ballisticast, IMHO they make the best molds. Gas checks are available from a place that I think is called casting specialties. A buddy ordered all the 50 cal checks I've used, and I have enough for my purposes. I think people originally used 1/2" freeze plugs for 50 cal checks, as they are much thicker then the smaller dia checks.

Start hitting up tire shops for wheelweights, there is no such thing as too much lead with the heavier molds, so if you're offered 10 5 gallon buckets of wheelweights, for $20 a bucket or less, get em all. Seems like more and more shops aren't selling their ww's, so it might take some time to find em.