I think it's apples and apples. My ref book stops at .250 (?) but a helical coil tap OD for that size is .316. Since a 5/16 is .0625 larger diameter one would suppose that the helicoil tap would be .0625 larger, more or less. Well that puts the helicoil diameter for the 5/16 a larger diameter than your .375 (⅜) tap.
If you don't get your question answered directly, I really doubt the castings are that close. These engines are probably "production jig" bored meaning there will be centering tolerances on the drilling jig meaning that extra meat needs to be in the bosses to keep from breaking out. No need to change the depth so you don't have to worry about that.
I think I would go with the 5/16 helicoil anyway and when you put it in, wipe the coil good with JB weld. That will help to seal any pores you may open in your machining process. The nice thing about the coil vs just drilling and tapping the ⅜, is the 5/16 thread is running in a stainless steel thread and the pressure of installation, is mainly on the coil, not on the block.....don't forget to knock the tang off the coil.
Or, just chase the threads real good and degrease it with something like spray brake cleaner and fill the hole with JB weld. Then drill and blind tap anew with your 5/16 tap.....I still like the coil better. We preferred them industry for the reason I gave you.
Also, I'd clean up the threads on the bolts on a wire wheel and use blue Locktite (thread sealer) on the bolts. It will lube the threads going in, help to seal the hole, and assist in removal (corrosion reducer-preventer) if and when you have to take it back out.
HTH,
Mark
