You might want to set a head on your block with the bigger valves in it first to make sure those valves will work on the smaller 4.250 or 4.280 bore without hitting the cylinder walls
Way better to be safe now than wish you had checked before having the heads modified with the bigger valves
i need to do the same thing on my 1969 dart GTS 383 motor after I have it bored to find out what size it will clean up at Luckily I have two sets of 906 heads to pick from, one with stock size valves and one with the bigger 2.14 and 1.81 valves