OK, after looking again this morning the motorhome 452 head has a tapered seat with 6 threads in the hole. The other 452 heads I have that I swapped on because they already had the springs I needed and a better valve job have a flat seat and 6 threads.

The 906 I checked the 3294s in last night had flat seat with 4 threads in hole and 3 threads showing in the chamber.

So yes, Im still not sure on the plug number I need because a 3/8 reach plug would be short in the hole to me.