That shouldn't move or flex... and if you rode home on just the front mount, then it's definitely strong enough, just gotta beef up the rear mount either like I did with double stock blocks or whatever else you can make work. The idea is to have both mounts strong and rigid enough to be able to hold everything without the need for the second mount, but use both mounts and that thing shouldn't be able to move at all once fully installed. When done, you should be able to push on the head from the side really hard and nothing should move. I've been looking for a way to make a head mount bracket to tie the engine rigid to all 3 frame tubes, which would make for an extremely solid setup. It would be real easy to devise a way to make a head mount with the head you're using but with a Fred head I would have to machine down 1 or 2 of the fins just in front of or behind the head studs to get a strong enough piece of steel or aluminum flat stock to mount to the head, and longer studs would also be needed. That would make the frame even more rigid and also dampen out even more vibration since all 3 frame tubes would absorb the vibes instead of the seat post and lower tube. A top mount would also have to be designed in a way that it could be easily installed and removed in case the jug ever needed to be removed while the engine is installed in the frame.