Here is one source of the head inserts:
https://www.pricemotorsport.co...ody_ins-351c_4v.html
per the instructions:
The kit contains 8 aluminum castings. The castings are glued into position with the Miller-Stephenson glue and left to sit overnight. Then you drill and tap and insert a button head bolt from the bottom side of the intake runner. Now that the insert is secure. Because these inserts are pretty much to size, you should make sure during installation that no part sticks out past the port surface.