450 HP on a streetable 383 is not going to be a problem. Lingenfelter's 383 made 420HP, 50 state legal with the little 211/219 cam.
Just remember you want a cam that is going to make the power in the 6500-6800 RPM range. You don't want to try spinning to or over 7K with a stock EMC and opti.
Depending on your budget, I good set of ported LT1 heads will work fine, LT4 would be better, and aftermarket even better. Trickflow just announced a 21 degree LT1 head that is suppose to have some big flow numbers? I haven't seen any real data on them though. I'd love to try a set out on my setup!
My own opinion is to just not go cheap on the valve train, spend the few extra dollars when you get the heads and go with, double springs, upgrade lifters, hardened pushrods, guide plates. With high compression and nitris you really want to control the valve train, and it sounds like you are going to be running a fairly aggressive cam for that much nitris.
What the heads will support is going to effect cam selection, but in general, ... If you want the car streetable, be careful with your cam selection, the stock ECM can only be tweaked so far before the cam surge will prevent the car from idling and having any street manners. You're probably going to want a cam in the 236-242 duration with a tighter LSA to bleed off some of the compression since you're planning a 200 shot and still make power in the target RPM. Some of the big nitris cams really need an after market EMC and ign. system.