Too much horsepower?

My heads flow a little less than yours (300@ .700, 2.055 valves), and my cam is an FT cam 251/257 duration with a little over .600 net lift. I have about 20 cubes on you, and mine dyno'd at 513HP, 526 lb/ft. I really don't see 25cfm more flow (with probably less velocity and more shrouding) and the cam difference being good for an extra 277 HP.
You said the block is partially filled and has a girdle.. I think it would be ok to run it. Limit the RPM's and adjust the throttle cable so it doesn't quite open WOT if you're really worried about it.