Captain232 - I think the design will work as long as the rails are strong enough. It's not shown on your image, but I assume you plan to bolt the firewall in place.

I don't like the design simply because it means I have to mount the engine to a mount and the mount to a firewall. Often I just want to check out an engine and might not even have a mount for it yet. I'd rather have something more universal that I can drop the engine into, such as my PSP test stand. But it will only work with beam mounted engines which is 95% of what I have.