Dumas made two versions of the .20 size deep vee. The original one was 24". They later introduced a "stretched" version designated the CF deepvee. I have built both versions in the 70's (or 80s). Let me know which version you have and I will try to find the instructions.
While these hulls certainly brought back a lot of memories, you may be disappointed if you intend to race them. Also, a modern .21 engine will likely be way too much power.
I suggest X-440 to start and run a strut depth at 1/2" to 1/4" below the vee.