One of the local racers built his race /6 with 318 valves in the late '90s. (1.78/1.50) but I don't remember what all he said was required to make them fit.
thats what SS is running but they are almost 5" long, 2 TENTHS longer... i dont want to run into geometry issues...
i found Ford 289 intake valves that are 1.78 intake head diameter, 4.865 long ( 1 tenth longer) and a .360 tip (.060 longer)...
so these valves allow a little taller valves without running into geometry issues, have the bigger head, and run a 11/32 stem so they take up less port... only thing is i may have to offset the guide a little but will have to see...