I'm guessing I have the most real world experience with the less robust frame rail style kits. I have the original APE frame rail kit. Imagine Carpy's kit without the cross piece. Just the 2 straight tubes with the 4 sliding sleeves. Big Jay at APE has said here that he sold many thousands of those kits back in the day. I bought 2 in 1980 or so. I installed one, the other is still new in the blister pack.
I used it on my bike as a roadracer, some drag racing, and then 50,000 miles (maybe a lot more, the speedo was broken) as a tourer, often 2 up, often overloaded. I suffered no frame wiggles or fatigue cracks or anything negative at all.
So, I also now have a Gordon's kit, for another project. It is far more robust, but may be more than is necessary, by my experience. I wouldn't hesitate to use Carpy's kit, or CycleX. Install and enjoy, no worries.
Realize if you will, there are tens (were) of thousands of CB750s running around with those tubes cut out and NOTHING replacing them. I've never heard of a failure.
