I have a 383 with about 500 hp and was thinking about going with fuel injection instead of my holley,will it make more power,better cold starting,any used ones around?

I'd look at the Edelbrock units instead. For not a whole lot more you'll get a port injection system that offers all the advantages of fuel injection.

Scott; A high duration cam will put a lot of unburned fuel in the exhaust. This will fool a computer system in closed loop into leaning out the mixture quite a bit. Might want to look into this before spending some big bucks. tom