I spent a year going through this. i had Certainbass build me a 5 string P/J and it arrived with the shorter P installed wrong so the BEA coil missed the A string entirely.
For issues I dont have time or space to post here-I opted not to ship it back to St. Louis to have it corrected.
After extensive research, it turns out (other than proprietary pickups wound by several luthiers for their own product) there are only four long P5 pickups available to purchase-the first two by FMIC which are Fender American P5 and Fender American Deluxe P5-then Wilkinson, and Nordstrand makes the correct sized coils for a longer P5 with no housing.
I opted for the Fender American Deluxe and had Jim Mouradian do the route and install in Winchester, Ma.
FMIC has the longer length P5 design on lockdown- they wont let anyone who is Fender licensed (read Warmoth) route for it, they wont let any aftermarket pickup builder build one for widespread consumer sales- hence why the only other two long P5s you can get are Wilkinson (China doesnt have to worry about licensing agreements yet) and Nordstrand (can get the coils but not the shell).
If i wasn't over my head in my own micro businesses it would make me want to start a fourth business in order to rectify the issue. My wife would kill me!
And one last thing- don't expect to open a real dialogue regarding this on TB- I tried in the past and no one seems interested in pissing off the Evil Empire (FMIC). Just my 2 cents.