This may clear up the question about holding the cells in place. It is a diagram that will be on the instructions

does the 7-cell lower plate fit the 1st generation gmaxx chassis?

The G1 and G2 braces will fit with either the G1 7 cell or the G2 8 cell lower plate (they have the same hole pattern).

The only problem you'll have is the slot for the motor mount was relocated in the G2 lower plate. If you're using the G1 braces you'll need a few washers on the back of the brace to attach the mount.

If this doesn't make sense - Maybe you can tell me what combination of parts you're asking about and I can try to clarify.