Try to find a cam with higher lift but lower overall duration, don't forget custom grinds, I would look for a cam with specs similar to this in your shoes.
Overall Duration should be around
Duration @ .05lift should be around
Lift is where this gets tricky....
Normally I would say look for as much lift as possible at those numbers, BUT you have factory pistons with no valve reliefs so lift should stay under .525 on the IN/EX valves, and even then clay the pistons and measure the thickness with the cam you have now; here's why, if you check the current cam setup (also using the new heads of course because valve depth may differ) and have say .100 (remember we are dealing with included angles between the valve and piston here so be ready with your trig skills) you could in theory get .020 to .25 more lift and still be at a safe .060 piston to valve clearance. Now for more math, if you change the head gasket thicker or thinner from stock, this will also affect the final clearance.
Now If you want to play it safe a cam like Lunati's 10230703 would be a great street cam make huge bottom end torque, have great mid range, and still pull well up top, but if you have the room Lunati's 10230704 would get my nod, if you happen to have miles of room, or get some pistons with a valve relief, you could throw some 1.6 ratio rockers on it down the road and bump the lift to .547/.568 lift to liven up the top end... a lot:)
I know a lot of guys don't like using higher ratio rocker arms, but I love doing it. You can get substantial lift gains with minimal duration increases. Going from a 1.5 to a 1.6 only adds about a degree and a half of duration overall lol, but could mean extra 30 to 40 horse if the heads can support it.