Having had more time to think about this today (hectic yesterday), and with quality being in mind, I am tempted to say that the insides should now be printed on a slightly thicker stock. I am now suggesting 115 gram, I believe it may be described as 80# in the US, this would need to be verified with the printers. Although 104 gram (70#) is adequate, I think a slightly thicker sheet will prevent show through ( text visable through other side of sheet). All this obviously will need to be discussed with the printers, presuming there is no sample sheets.
I should also point out that some printers will use a 104 gram stock as an alternative to 115 gram, nothing wrong with that as long as it feels about the same in hand. All down to paper density.
Sorry to complicate things a little.
To sum up, the insides should be printed on the thickest and best quality stock possible for that particular type and size/thickness of book. Something may have to give somewhere, depending on price.
Err on the side of caution, is whats in mind.
Alex.