If you look up Gordon Whitehead from the UK you will see he designed an outstanding P-38, I think his plans were sold in RCM or MAN plans. I haven't built his P-38 but I did build one of his Krier Krafts and it was a great build and a lot of fun. Some of the stuff was pretty old school so it left a lot of room to redesign for items we have today. It's a lot smaller then the Giant Nick designed and his plans are a lot cheaper. Smaller also means less wood and cheaper hardware. I think I would give this one a try before I tried to design something myself just to get my feet wet. I have seen a lot of Nicks P-38s over the years at IMMA events, some more scale then others but all of them outstanding planes. You do know you can just buy his plans too?
If you go into the scratch and designing forum you can see what some of the scratch builders have to go through to design there own. Today it's done with a CAD on a computer, I used to do it on my drafting table but find it easier to just buy someone elses plans these days.
I have given some thought to building Gordons 38 myself.