I put a 700r4 in my 72 c-10 and I love it. In order for your trans to truely lock up in 4th gear and get that better mileage you have these two options down below.
The first one is more expensive and causes the converter to disengage upon braking, whereas the 2nd one will also lock your converter in 4th gear but does not disengage the converter on braking but this one never gave me any trouble so far, beside the trans returns to a lower gear anyway when you slow, stop, or deck the throttle.
The choice is yours either way you have to drop the pan, do yourself a favor, add a drain plug and have it welded (the nut the drain plug threads into) on from the inside. Change the trans fluid 30,000 miles or less and when you take off the filter be sure the small o ring gasket is not still stuck up inside the trans, use a small hooked tool to pull it out evenly a little at a time until it come out.
Good luck.
http://www.speedwaymotors.com/700R4-...-Kit,2362.html
http://cgi.ebay.com/ebaymotors/LOCK-...7#ht_777wt_877