More Front Runner goodness...
First, I've remounted my SlimLine II rack using track mount feet from Front Runner.
I originally went with the Yakima setup because I had it already and based on information available at the time, it was unknown if the FR feet would work with my setup. I only had a couple of days to get the rack installed and other things sorted out before we left on our trip, so I didn't have time to look into it further at the time.
Now that I've had some time to revisit this, I've been able to determine that the Front Runner feet will work splendidly for my application!
The 1425mm(W)x1358(L) SlimLine rack combined with Front Runner 70mm track mount feet is EXACTLY the right width to mount on my TCUSA tonneau's rails. With the feet pushed out to the widest possible mounting, right up against the rack side rails, it's a perfect fit. Not a mm to spare though...
The Front Runner track hardware is more heavy duty than the Yakima stuff but won't fit in the tracks that came on the cover, so I used the Yakima "track nuts" and some SS bolts from my stash. The bolts I had on hand were about 2mm too long and bottomed out in the track. Rather than order more hardware or stack washers, I machined some Delrin spacers to take up the slack.
The net result of using the Front Runner is an overall better mounting solution. The FR feet eliminate the bit of flex that the Yakima stuff had due to their abundance of plastic parts and is an overall cleaner and lower profile setup. I did lose the QR feature of the Yakima towers, but I don't think I'll miss it.