Remember...the springs in PI heads can barely take the stock cams. If you want bigger cams, even mild ones, I recommend new springs. You'll spend every bit of money on those heads. I work at a shop, and to do my heads up (p&p, cams, springs, and all the stuff...gaskets, bolts, etc.) it would cost right at $1800-2000. And that is my price( I don't get much of a break)...customer would easily be $2500 up. Labor to do heads is 14 hrs...at whatever the shop rate is...65-80/hr...you do the math. Add 100 bucks for new bolts (old not reusable), 100 bucks for head gaskets, whatever for porting, etc.. The list goes on. Doing the heads on a 4.6 is not really a driveway job like on a 5.0. Either save some more and get a charger, or get some nitrous. The NOSzle kit would be the way to go. N/A tuning works better on 4V's in my opinion.
Or, disconnect the front sway bar..that was worth .2 on mine, .5 on my cousins 347. Gears like 4.10 or 4.30 and some
lower control arms will help. That should put you in the 13's.