The short answer is yes , but you figured that out yourself. Early engines used the thrust plate to seal the passages. Their thrust plates were machined flat to seal these passages. New thrust plates may not seal and hence the need for core plugs.
Thanks, its the answer i was looking for. But not having a great deal of experience on these engines i wasn't sure what to do. The core plug kit i got from Mancini (or maybe Summit) had these two plugs in it. So i assumed they had to go in; - plus i was reading "How to Rebuild Your Small Block Mopar", by Don Taylor and Larry Hofer and it said to install them. Although i have since found a few mistakes in this book, one of which is on the same page, saying that the tin drip feed for the timing chain, if used, has to be put in the same position as the bolt oil feed method, ie top RH side (or left side looking at the engine). This obviously won't fit here (i tried!), it has to go in the two lower bolt holes.
Yes i have now been told that most engines didn't use the core plugs, the machined thrust plate must have been good enough to effectively seal the galleries. And if you had to use a new replacement plate that was not well machined then the core plugs would then be needed. Thanks
