If it was me ( and it could be ) I would put the 513 pickup the the bridge position that way you still have all the goodness of the 305 bridge pickup and something more when you need it.
A Super Strat version of the 305 would be killer and a don't believe it would effect the value it might even increase it if done well.
What I would do is basically add a Blower switch ( Push/Push Pot ) that adds the extra coil to ANY position of the guitar.
But where do I buy a 513 pickup?