I never rev my 305 past 3500! I wouldn't dream of reving a 478 past that either. haha
Another thing I was considering was fuel injection. The MSD injection is spendy, but a sweet unit. Be great on a motor that size. I've been wanting to make headers for my 305 for the cool factor, but haven't gotten around to it.
Maybe I'll just do nitrous on the 478.