I did my 700R4 a couple of years back now. One of the best moves I made. With 3.73 gears in the Lukester, the low first gear is insane and costly for my tires. At 70 MPH, I'm turning about 2300 RPM as opposed to about 3400 RPM with the TH350.

This is my decision.  I have talked to a very reputable, local  transmission person (who happens to be in the process of restoring two sharks), so he is very aware of a C3.  Because I don't do very much freeway driving, I have decide to go with a set of 3.08 gears.  It was calculated that while dring at 65mph, I should be around 2550 rpm's (compared to my current 3200 rpm's).  I can buy a set of 'Yukon' gears, and the other items for $440.  The mechanic has not gotten back to me yet with a labor fee (this same guy rebuilds differentials too).  I am going this route because this is the least expensive and I have much more to do (radiator, A/C, brakes and finish chasing that oil useage issue).  If I had gone with a different transmission, I had decided (for me) I would have gone with the 200. 

Dave
Glad to see you have made a decision on which direction you are heading. My 74 has a TH400 with 3:08 gears. I have no issues driving it on highways and I think you will find there are a lot of vettes that have this gearing with an automatic. Are you going to get another carrier to do your gear change or are you removing your old gears and replacing them with the new gears using your current carrier. I only ask this because you can probably find a set of 3:08 gears and then keep your old gears in case you would have to go back to them. This is what I did with my 3:08 gears although I dont think I will ever be going back to them so they sit here on the floor going to waste. To bad your not closer. If you want an actual RPM reading in drive at different MPH I can give you what mine is.
Best of luck with your conversion.
Rodney