Are hardened valve seats necessary?

[Jac Mac]

I would give those CHI heads some more thought for some NOT so normal reasons.
The std 351c 4v heads have 1.71" dia exhaust valves, when you install hardened seats to accomodate these the O.D. of the new exh seat starts to encroach into the intake seat area in an area which already has very little metal due to the large valve dia's. By going back to the 2v exh size of 1.625" you can help this situation a bit, but then your up for new valves etc as well.
In years to come your current 4v heads will appreciate in value to collectors with race cars etc who would probably prefer them unseated for use with race fuel ( you may even be able to get more for them in Australia than the USA )
When you take all that into consideration the CHI heads may be the way to fly. I have built one motor for a circuit car with them fitted and there is absolutely no way we could have acheived the same performance on iron 4v heads that this car has. ( And the owner had a set when we initially started the build ) The increase in bottom end torque without any penalty at high RPM, plus the weight loss in regard to handling made it all worthwhile.

Jac Mac