400-600 grains @ 2000 is about NFB to full nitro give or take.

My first DR build was also on a Spanish gun, however it was a 31/2" magnum origionly proved 25% higher than this one.
I elected to use 450#2 for the low pressure it generates. There is a pretty extensive thread on it if you care to wade back through my posts.
Don't put sleeves in the barrels, this is not the way to do it and will anyway give you nightmares on regulation. Cut it too a monoblock.

If it is to be a fun gun for you. Then I think that I would be tempted to make it 450 31/4" but only regulate for NFB loads 365-400 @ 2000-2100.
My reasoning would be that it is much more plesent to shoot for regulation, will kill anything in Canada or indeed Africa ( except the big three) with authority. You can shoot cast lead which will encourage its use more. You can build it lighter.
The bottom line is that it aint all about chamber pressure. Whatever power a gun generates at the muzzle a similar ammount of energy must be absorbed by the stock and action on fireing.

I would look at the diamiter of the firing pins, their fit in the disks and their protrusion and I would look at the disks themselves as the wrench holes look quite large and close to the primer (but this may just be photo angle)A pierced primer is one of the major things to be concerned about in a DR. Look at a BA firing pin size tip and protrusion.

Before you start. Is the gun tight. ie on face and with no rock in the action if you take off the forend and attempt to twist the barrels in the action. If it is loose now it will not get better, so don't begin.
Building a DR is not like building a BA. The most important single thing is that you must be able to solder properly. If you cannot now, you must practice like hell you will do a lot of it and the integrity of the rifle will depend on it.

Hope this helps.

Regards