I used the John's Car mounts and found them easy to install and the quality to be good. I could have made my own but my labor to dry fit the motor multiple times, buy materials, weld, re-weld, fit, re-fit etc. made it a simple decision to simply buy from JCI. From start to finish it took me about ~3 hours to install my LS1 w/ 4L60e trans with the JCI setup. I also bought John's headers and all other associated accessories (once again good quality and all the "homework" is done).
I happened to be in Dallas, TX last week so I stopped by JCI to meet John. John is a great guy who provides good support, willing make things right if there is a problem and willing to provide information if you have a question.
The only caution I have is if you are installing an automatic transmission... I bought the JCI trans mount and found it not to work well with exhaust fitment using a 4L60e trans. It appears it would work great with a 6 speed because the 6 speed is much narrower in size but with the automatic it's not conducive for easy exhaust fitment. On the automatic I found the mount to be positioned too far forward not allowing sufficient room for the exhaust to pass by the rear of the trans pan and through the exhaust locations on the mount. As a result I modified the JCI mount to provide more room for the exhaust.