Battery question

[FG101C]

I hate to start a new thread but can't seem to find an answer to my question.

I decided to convert my Baja, DDM had the kit for $475.00 w/free shipping last night.

I recently ordered a Nano-Tech 5s 4000mah 45~90c pack but it won't fit the original application.

My question is, and it seems like this would be fine, if I get the same pack, same mah and c rating but 3s I can run that and the 5s in series for 8s correct? I mean the length and width dimensions are the same for both packs so that tells me they are the same cells just one pack is 3s and one is 5s.

What do you guys think?