Looks like your MY6 uses a similar speedometer gear carrier to the NP208 in my 76 & M1008 and some, but not all, TH350 & TH700R4 from that timeframe.
There's an O-Ring round the large part of the housing and a seal around the cable end of the gear. Either one can leak but usually it's the seal inside the threaded end around the gear stem.
http://www.ebay.com/itm/like/1511809...&ul_noapp=true
I believe part #s are as follows
12440382 SEAL, SPEEDO DRIVEN GEAR (the orange seal in your pic)
1361618 OIL SEAL RETAINER, SPEEDO DRIVEN GEAR (C-Clip in your pic)
I believe it's # 326563 SEAL, SPEEDO DRIVEN GEAR SLEEVE (Black o-ring in the gear carrier)
You can find the orange seal online and probably locally.
The GM 43 tooth driven gear is Purple and probably around $25.
More than you want to know about speedo gears in the 73-87 trucks.
http://www.73-87.com/7387garage/drivetrain/speedo.htm