[theOZer]

James,
Yes, this mod works in vB3.7.4, also in vB3.8.0 betas & RC1.
After reverting your threadbit template(s) to vB374 did you re-add the thread thumbnail code to the threadbit template(s)? The threadbit template IF statement you need to update is a bit different from vb3.6.8's threadbit template but you should be able to figure it out, where to add Jason's TT code.
If you have the 'Thread Thumbnail - Search' plugin turned ON - Did you add the TT code to the vB3.7.4 search.php file? (see Post # 112 on Page 8, the change also applies to vB374)
If you have the 'Thread Thumbnail - Tag Search' plugin turned ON - Did you add the TT code to the vB3.7.4 tags.php file? (see Post # 169 on page 12, this change also applies to vB374)
I have a bit lengthy (too lengthy to post) write up on installing just the forum Thread Thumbnails mod. I can PM you it if you're interested.
Regards, theOZer