Since using huge D6s is one of the details I'm looking for in the rules, one of the obvious candidates would be Swords & Six-Siders, which sports a strong OSR feel while keeping all rolls down to a single D6.
I'm sure there are other D6-only OSR efforts out there that follow in the footsteps of classic editions of D&D...this is just the main one I'm familiar with. And while I've never played it, it looks both solid and fun. (I'm also a fan of the generic OSR house rules for D6-based combat given in this blog post over at The Bogeyman's Cave.)
Some big advantages of going this route include the use of the OGL (and S&SS has an accompanying license, I believe) and the huge amount of material that's out there to
On the other hand, I probably don't want to use granular hit points in a big outdoor minis game...and I think that going this route might tempt me to also make use of a giant D20 for combat, rather than just the D6s. That could end up being some combination of expensive and a hassle.
Still...all in all, it's a nice option to consider...