how would you get a p body with a j neck? would you have to special order it at a cost to you? do you buy an afterstock neck and forfiet the fender logo and authenticitty? do you buy both basses and pawn the j&p off on ebay?
Yes. Seriously, you've hit 3 options: Fender custom shop; aftermarket neck; buy one of each and swap necks. There are more options: aftermarket body. That way you keep the decal! Also, some older P-basses were made with "A" width necks. Of course older Fenders tend to be expensive.
I found a nice Fender MIM Jazz neck on Ebay and put it on my P. There are usually some there, do a search.
i'd say ask warmouth directly or ask nino brown. i'm thinking that there should be a bit of a mismatch in the width of the base of the necks but i'm sure there is a standard way around this if it exists. It's a pretty common mod, even duck dunn had it done (ouch ) recently for comfort reasons.