Just found a pair of ST-r500 flight deck shifters on Craigslist for super cheap (maybe a divorce or a rich person's old take-offs??). Anyway, their loss is my gain, right?

I bought most of a 7400 group back in February (2x8), and I was thinking of replacing the downtube shifters with these. Can anyone think of a reason why they wouldn't work? I saw a review on Amazon that said a guy had trouble getting the shifters to reach the last 2 gears on his 7403 group. I think I am going to buy these either way, but are there any thoughts on the subject?

I am surprised no one answered this back in the day. The shifters will not work with the 7400 group because there is a difference in the cable pull required to shift between gears on the pre-1997 (i.e. 6, 7, an 8 speed) Dura Ace rear derailleurs.

See Sheldon Brown's site Google search for sheldon brown dura ace pull ratio The forum would not allow me to post the direct URL to the site because I have too few posts.

https://www.sheldonbrown.com/dura-ace.html

Oddly enough, when I bought my Zunow Hummingbird several years ago, it came equipped with that exact combination: ST500 STI's and a 740X RD. Much to my surprise, it shifted just fine when I tested it prior to tearing it all down and putting my own components on it. Used the standard (not optional) cable attachment point as well.

No, I cannot explain this.

9 speed shifters work with Dura Ace 7400.
Sl 7700 shifters are 8 speed 7400 or 9 speed 7700 as per Shimano docs.
All 9 speed shifters are the same pull.
So any 9 speed shifter will work

Yup, B-routing of the RD cable where it's clamped. One of the very few OEM backwards compatible efforts that actually worked properly!