There is no denying the satisfaction of shooting your "own" rifle.
It is probably cheaper to get a small caliber DR from Europe than to build one. However, it can make sense to build a large caliber one. The volume for european DR is entirely in the small calibers which explains why their large calibers one are much more expensive and probably not as well fitting as well.
Hence, I believe that building a large caliber makes sense, not only as a personal challenge, but on a practical level as well.
As pointed out by 400NE the interest in the 450No2 for a shotgun conversion might lie more in its "my round is longer than yours"

characteristics than in actual pressure numbers.
The 470NE (for a 16GA base) is probably the best choice IMHO because of availability and cost of reloading materials, with better resale value to boot. The 500/465 follows closely. Otherwise, the 577 3" NFB would likely be better suited for a 12 Ga base.
I have been looking hard for a suitable base for such a conversion, without too much success so far.
My personal wish list is:
- 12ga 3" or heavy 16Ga
- boxlock or sidelock back action (one can dream)
- long action bar
- bushed firing pins or small firing pins
- third fastener
- Pistol grip stock, heavy and large at the wrist
- monobloc or chopperlump (see #2)
- low cost (see #2)
- pitted barrels to help the previous one

I have yet to find the right candidate !