Best head for the money??

of what you have, use the 906. the 516 will help compression but they just dont flow. i wouldnt put a ton of money into whatever ehad you choose. i wouldnt even worry about hardened seats, it will take a lot of miles before you have any problems from unleaded gas. just do a good valve job. im guessing it isnt going to be a 20,000 mile a year daily driver. i had a set of 906 heads rebuilt in 2000 for the 431 in my dart. just decked for a flat surface, and a good 3 angle valve job original valves. i went 11.80 with them and drove it quite a bit. i put a set of stealth heads on it and disassembled the 906 to change the springs and just check them out, they looked great. i put them on the 400 over the winter and put it in my duster and it runs great, put over 4000 miles on it since the beginning of june.