One of the problems with a preliminary sonic check is it's just an indicator...it doesn't tell you what will happen after you bore the block...pin holes can show up and other stuff.....the potential for years of water damage and rust damage on the back side of those 390 cylinder walls is GREAT..
perhaps a picture is worth a 1000 words...
Typical FE Cylinder wall
Another view of FE Cylinder Walls
So when you think of cylinder wall thickness you need to also consdier what will show up that the sonic check can't always pinpoint ahead of time...
more FE wall problems
My last comment....
(machinist said it needs .030" to .040" to clean it up due to a few pits in the #1 cylinder),
Listen to this machinist....because he's protecting himself....the potential is great for additional problems to show up...you can see the pin holes in the pictures above....my advice is if you want a 427 bore start with a 427 block.....
The 390 can build a great engine and some of the ideas above are possible.....
Not trying to cause a prblem with this post but if you do bore this block and things start showing up then you'll be sleeving every hole to be safe....then you'll end up like this image...
One last warning....
George