couple of things the wall wart charger will take forever to charge the batterys your talking about, and may not charge 6V batterys at all, your better off getting a quality charger.

making battery packs can be tricky, these days you can pick up a Nicad or NiMh battery with huge capacity for cheap on ebay,or from www.cheapbatterypacks.com I just bought 4 NiMh 4.8V 2500's for 30$ off ebay, well worth not having to hassle with making them http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT .

the same seller has 6V as well if thats what you want its up to you