i'll get the valve seats question for you... its a questionable thing i've ran soft seats and never had any problems, the lead was used as somewhat of a cushion for the valve seat to keep it from being damaged, it also kept the octane higher to reduce pinging... thats why a lot of the high performance setups in the 60s run like crap on todays gas, the lowest octane then is higher than our highest octane now. if you were to do a full rebuild i'd say go for it.... if you were going to have head work done, i'd say go for it..... but for the time being until you are going to be doing something anyways leave it..... just make sure to run the higher octane fuels and if you can find it, run lead additive every few tanks....... having hardened seats done is generally expensive, most shops will charge you around $400 to change your seats.... and the labor is whats expensive the seats them selves run under $2 a piece... i dont know about your heads to intake question though, i'd check what port volume is on the intake runners on the heads and the intake, if they're close then they can be compatable......