Your question last year and the replys got me to design and build a fixture similar to yours. I use 4 posts to support the head on the headbolt washer surfaces. Instead of splitting and expanding the tops I use a tappered pull down fixture thru the sparkplug hole. The plate is 1" and the posts are moveable to do both front and rear heads on the same fixture. It works very well.
Yours looks real nice. Great idea and execution.
Thanks for getting me going on mine. I've used it to cut pop-ups in to heads, make #6s out of #5s and true up WR heads. 'dog