Reserve the coarse Whitworth system for threads directly into alloy, this thread form gives good support to components mounted in a relatively soft casting. However in alloy it will will not support the loads exerted by head bolts*, hence the use of inserts to spread the forces.
For Alloy heads, preserve the original cast in insert if you can. For a Helicoil type re-thread, choice at 5/16" is CEI at 26 TPI, BSF (22 TPI) or UNF at 24 TPI. So with the Helicoils available 5/16UNF would be a reasonable choice. You'll need bolts to suit, but these should be easy to source, and take care with those blind holes, a little customising may be needed for the correct bolt length to prevent bottoming. Are you sure the bolts you have are correct and unworn? A new set of bolts to fit the existing inserts may be all you need.
Swarfy.