If you are looking for a boxlock the strongest is going to be a Greener Facile Princeps. I'd love to do a conversion on one.

Other actions include Simson Suhl, they can be had for under $800. If you want a sideplate gun you may tru to find a "BC My Luck"(now BC Miroku) they used to produce a H&H style action, they are hard to find though. H&H Dominion actions are also great.

For something different there is a Husqvarna hammer gun with Jones underlever on GunsInternational that could possibly be converted to a double rifle, only thing I wonder about is the thickness of the standing breach. Otherwise it a very classy gun.