Is there a significant gain in flow to be had from moving from the small valves to the big valves with these heads?

 

Which heads 307 or 350?

 

Sorry. For a 1972 350

 

Yes, it definitely helps, think of the W31 with 2"/1.625" valves. Of course the bowls need to opened with a cutter at minimum, even better with a grinder to transition into the port.

 

What cost is typical for having this work done?

 

I depends a lot on the shop rate in your area and the condition of your heads. Also make sure enough clearance is reamed into new valve guides, machine shops seem to put them in with not enough clearance on the center exhaust guides. If the guides aren't all wasted, it helps cost. I think a pair runs from about $800 to $1200 depending on the extent of the work. Mine #6 heads were $400, fresh machined with big intake valves bought them unused and fresh machined off Kiijji. Up here in Canada since stuff costs more, I was told they were $1200 worth of work, I would believe it. The cost is close to a set of Procomp heads but you need an electric fuel pump and screw in studs, guide plates and roller rockers to go with the aluminum heads, along with at least a .030" mill to reduce chamber volume for a 350. Also cheap Felpro head gaskets aren't recommended with aluminum heads so add another $150 for Cometic's over Felpro head gaskets.