I'm think of eventually usiing the SC module yet setting in the FR. Wher would be the best spot to set it up in that world?
hmm...
SW side of the realms, near Chult?
I DM'ed this AP a long time ago in the Forgotten Realms. I placed it in Turmish, a small country enclosed by moutains on the northwestern shores of the Vilhon Reach.
I used the city Gildenglade on the verge of the Aphrunn Mountains as my replacement for the Shackled City. The official background for Turmish allowed for the SCAP setting, but was also vague enough to give the DM ample opportunity to adjust it to his taste and to the AP.
Considering that Cauldron is supposed to be in a tropical climate, on a volcano caldera in a jungle/rainforest, and with the sea not too far away (Sasserine from Savage Tide AP is supposed to be the closest port), I'd used Chult.
Specifically, I'd use Luo Peaks (the mountain range going from the SE end of Lake Luo), and put Cauldron about halfway along the chain. That gives you enough tropical rainforest to the north before you hit the Shining Sea (where you can insert the port of Sasserine).
This has the advantage of making it easier to run Savage Tide later. Sasserine is already in place, and the long sea voyage can go around the Chult peninsula before heading "off-map". Plus, your players will already be familiar with the region.
Here is where you can find the relevant document to convert Savage Tide to the FR (Savage Tide Conversion Notes - Here There Be Monsters). Use that as a starting point for your own conversion of SCAP to the FR. (Note that my suggestion above is not precisely where the conversion document would place Sasserine and Cauldron.) (Also note that there will be a few spoilers for STAP in the conversion document.)