I also have a 340 with a casting date of 1969. I'll be looking to sell it soon. Just like you, I want to get a fair price and don't want to give it away.
The FBBO forum has a section where you can post thread asking 'What's It Worth?" I wish they had that here too. I see ads where guys list some stuff for WAY higher or lower than what I have seen listed elsewhere so a forum for guidance on pricing would be helpful.
For example, I have a 3.55 3rd member that I'd sell but the last time I sold a 3rd member, it was a 2.76 and I sold it for $100. I figured the 3.55 was worth more given the ratio and several years passing. I figure the 3.55 might be worth $200 but when I posted about it at FBBO, some guys estimated closer to $350. Wow!