I did something similar to that to get the shaft lined up with the cowl.

I mounted the engine on the mount; kinda screwed that up. I tried to leave room for the carb in back, but forgot to leave room for the mount bolt heads. [:@]

I then sat the engine and mount on the firewall, holding the plane vertical of course. I put the cowl on the fuse and just move it around until the shaft sat in the center of the hole. I did have to cut some area away for the head before I could get it work. I also put a spinner on the shaft to see how it would look.

Here is a pic with the spinner sitting on the shaft and then the final mount of the engine and cowl.

I haven't put the time in I thought I would so it's still in progress. So I won't get to fly it this weekend but I may next.

Let's see, I have to put on:
elevators - stab is glued in
rudder - easy CA hinges
linkages for the control surfaces
belly pan
canopy

Sounds simple... key word is sounds.