Did that years ago, I was able back then to get them at the super shops performance store.
That is a good thing to do to rid some of the heat soak. I recall I had to add longer studs and just do a slight tweak of the throttle cable too. This was on a 4 speed Challenger. I went to school with this guy back i nthe day and he was the 1st owner. We saw about 8hp at the tires improvement when the engine was hot. Also seemed to help stop issues with heat perculation as well.
I added one to my 340 Cuda 4 v carter recently too.