Motor instal

[dirtscoots]

I’ve got a dumb question. I’m definitely in over my head on my blazer, but I’m still attempting to plug away. The trans and transfer case are out being rebuilt. I am attempting to instal a new motor. I have new motor mounts, motor in and tight. It rocks back what seems way too much. Also my drivers side header is touching the frame it wasn’t before. My question is will the motor not sit correctly without the transmission supporting the rear? Any insight would be great. I attached a couple pictures. My goal is to get the motor dressed and start putting the front clip together so hopefully ready to fire when I get the transmission back.
I apologize I do not know how to make pictures show upright!