I am in the process of rebuilding my 427 sideoiler.
I have done several of these engines since I am a little hard on them. I have a 671 blown and Hilborn injected '57 Ford Ranchero drag car.
Your issue with the 1/4 NPT and or 3/8 NPT is interesting. I do believe the 3 threaded holes, 2 in the back of the block and 1 in the front of the block, where Ford drilled the side oiler passage are not NPT (Pipe Thread). The 2nd one in the rear is for the relief valve. I have a bag of Ford OE plugs, maybe 25 or 30 plugs. There are 2 types of bigger plugs (smaller NPT plugs are for the 3 side oiler holes on the drivers side of the block and numerous other locations on the block). As far as the bigger plugs one is NPT the other appears to be a straight thread. I believe these 3, straight threaded plugs, go in the front (1 hole) and rear (2 holes) of the block. I also believe Ford put a 2nd, very small, allen type hole inside the big allen hole in the plug. Maybe this was done to identify the plugs. For sure the NPT plugs don't fit these holes. Way to big.
Lastly I have searched all my Ford books and the internet and cannot find any info on these plugs. You do find info on how to correctly install the relief valve but nothing on the correct plugs to use.
Tommy T
