You're talking about Freeze plugs? If so then just remove the old ones and go to the parts store and get some replacements. The dimensions are right in the center of each one so be sure to not damage at least one of them so you can be sure to get the correct sizes.
even if you do damage them, just go to the parts store and tell them that you want a core plug kit for a 390 (any year) and it will come with all of them you need and then some, it should cost about 4 bucks
I'd be scared of screw in...if by some weird chance the tap breaks, or cuts crooked, you could need a block...I would go for brass if given a choice- they might not last any longer(dunno?), but I would think the softer material would knock in easier- less risk of hurting the iron...
thought: IF someone were to tap out their block for screw in freeze plugs, I would say the block stresses would be bad enough to weaken it, and possibly ruin their 'torque plate honed' round cylinders from the stress of a tapered pipe plug being screwed in- Or am I way off and screw in are actually straight thread/o-ring types??? never actually seen one out of a block.
edit- btw- often replacing these 'in the car' is nearly impossible- if you cant get to them you might have to resort to the (yuck) rubber type with expanding screw/nut...they work, but Ive seen some not last but a few years...Dad had a challenger w/318 that garage put these in- cheep fix, but started leaking after couple years- easy to swap for sure though.
<font size=-1>[ This Message was edited by: ford4v429 on 4/18/06 9:58am ]</font>
Are you talking about the rubber expansion type of plug? Those have a screw in the center that you tighten to expand the rubber . They will not crack your block.but...... they are not the best. Ive used them when I had no room to drive in the brass type. If you can just install brass ones .