Right, sounds like you could use a 5831. when searching for them you may find a letter after the model number, that will designate a different ratio for both low and over, or low and lower as some are. They always have direct.
Some info and links about them here
http://www.4btswaps.com/forum/showthread.php?14140-5831-b-ratios-%28Auxilliary-Transmission-Info%29
The 6041 is a 4 speed, two lows, direct, and an over. Ratios are 2.14, 1.24, 1.00, and 0.86:1 for 1st through 4th gear respectively
I don't know of any models with two overdrives in the auxiliary.
little more info on mine
http://www.4btswaps.com/forum/showthread.php?14924-3-53T-Silver-For-a-1952-Chevy-1-ton-pickup/page4Shifter could look like this and be mounted centered directly behind the main transmission shifter.
https://picasaweb.google.com/lh/photo/vbs2LCCJzp4rbpykXRModNMTjNZETYmyPJy0liipFm0?feat=directlinkYou do NOT want to leave it in OD all the time. The original literature and recommendation was to only use OD in the auxiliary while in the higher gears of the main transmission; so as to limit the total amount of torque you could put through the OD gear. Ideally you'd leave it in direct then shift into and only use OD as needed on the highway. You can also use any of the auxiliary gears to split gears or otherwise make for more progressive shifting with the main transmission, still avoiding OD while in low gears of the main. Figure out the ratios and which shifters to move where when.
I don't have any extras. These things are perhaps not expensive at all if/when you find them, but finding them is the key part... Therefore holding on to what you want/need is worth more than the search to find it again, if you even get that lucky.
