Anyone know for sure if I have to replace the stock valve seats if I want to burn unleaded fuel?

Depends on what valve seats you currently have in the car. If you have the brass seats, then you'll need to replace them with the steel seats.

I have an original motor for my '71 240 (1/71) that I'm rebuilding right now. I thought I would have to replace the seats on mine, thinking I had the brass ones. After pulling the valves, it turns out that I already have the steel seats in the head (E31).

You just need to check and see what's in there already.

As far as I know, the N-42 was the only one with bronze seats, however, the steel seats in the other heads woud not be the hardened seats. The softer steel seats will last quite a while, but the only way to tell if they are good or not is during a valve job....

Thanks for the help. I'm in the process of buying an E31 head to replace the one on my 240. I guess I'll wait and see which seats are in it. My E31 has been butched twice, so when this one came up I thought I should buy it. It's been pressure tested and magnafluxed. Do you know if it is easy to tell if the original seat are still in it? Are the stock ones brass or steel? I was told they were brass by the guy who did the first valve job.