No reason why it won't work. http://www.homebrewtalk.com/f11/how-...-needs-123585/
. Note you can only hit a 1.059 OG brew with a 5 gallon....of course you are at 6, so I guess you can hit around 1.071? (if my math scales)
I'd figure out some way to insulate your new mash tun....some high temp insulation, or low temp insulation that you can easily remove, would help you not lose temps so much, but you might not care so much if you are doing step mashes.
Out of curiosity, why so serious about step mashing? With 95% of the grain you can buy, you only really need a single infusion at a single temperature...(no need for the complicated rest schedule that less modified malts call for...)...