Great idea, CurtisFox; do you know what the min/max dimensions on these exhaust manifold spreaders are?
I used one of these I had laying around:
It was a tight fit to start with and I used a mallet to force it into the gap before spreading.
Be sure to lock the axle-end by replacing the axle and nuts before spreading! I found the job needed a LOT of force. 6061 Aluminium isn't very malleable, so you'll need to over-spread a little to get it right.