Glad to hear your first "voyages" on the Gemini were both fun and successful.
I use a 40 cm Design Lessacher Formula Weed on the Gemini to avoid the interference between the fin overhang and the rear end of the CB.
The Lessacher fin has no overhang.
If you are in Hatteras this weekend, I'll be down and you are welcome to try out my
40 cm Lessacher Formula Duo.
I wish I had a Gemini to measure, but I'm not in Hatteras and the only Gemini I know of is the rental board at Sailworld Hatteras.
What is the dimension (inches or cms) from the front of the fin box to the rear of the CB slot.
I'm thinking that maybe a Tangent Reaper 42 might have less overhang and would work OK.
I always used the Lessacher 40 cm Formula Duo as it's all the fin the Gemini needs, even with 9.0 m2+ rigs on the rear.
As far as the theads, the Curtis Someweed probably has 1/4"-20 tpi UNC threads in it.
Most were made with 6 mm-1.0 metric barrel nuts, but some did have the 1/4-20s.
It's not a real good idea, but you can tap the 6 mm x1.0 metric inserts out to use 1/4-20 fin screws. There's just enough diameter left in the barrel nut to get some about 1/2 strength 1/4-20 threads if you get a 1/4-20 tap.
Better to check with Gorge Fin Co. (used to be Curtis Fins) to get some metric barrel nuts for your big Someweed. Get the new barrel nuts and use an appropriately sized round punch to drive out the old barrel nuts and insert the new metric barrel nuts.
All Starboard fins come with 6mm-1.0 pitch barrel nuts and all Starboards (except "A" box boards) come with 6mm-1.0 pitch fin screws.
Check this link on the fin screws:
Hope this helps,