If you are looking for pressures, try this:
http://www.imrpowder.com/data/rifle/index.php

While not the SAAMI max, it will give you some idea.

Here is some one who does conversions for about $650 if you supply the donor gun.
http://www.angelfire.com/sd/rgs1/work.html

Inserts
http://www.mcace.com/shotguninserts.htm

The problem with the above inserts, is that there is no way to adjust the POI.

It would be cheaper to buy a Valmet o/u if all you want is a double rifle. If you are thinking of an African cartridge (375H&H, etc) sxs, then you will need a big war chest.

Pete