Flathead Relieving and new valve seats?

This may be a stupid question but, what do you do first relieve the block or put the new 1.6 valve seats in first. It is a 51 aba. I am thinking the seats have to be put in first so you know how much to cut for the relief but nor sure. First flathead and relieve job I have done.

 

Yep I would install the seats first. But why seats? If it is a block that does not have seats just open up the seat area to fit the larger valves. Block that now has seats proceed.

 

The block does have seats right now but are corroded bad and worn. Thank you for the clarification.

 

Depends on how you will do the relief...If you do it in a mill, then it doesn't matter.
If you do it with a die grinder and carbide burr, I would do the relief first with the old seats in place. If you slip and gouge a seat it won't matter.