Roll them up in corrugated cardboard, I use at least 4 layers. If shipping assembled with valves and studs I use either a wood stick or tightly rolled-up cardboard to add support in that area, as seen in the photo below.
Shipped numerous sets this way, never an issue.
I recently shipped a pair of bare '69 RAIV castings and they arrived in perfect condition as expected. This method works great for both $300.00 & $3,000.00 sets of heads, use it with confidence.
