If you get one that's built right it would be a nice upgrade.
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Definitely have it built. I recently swapped a 700r4 into my 82 K10, with new hardened steel sunshell, extra heavy duty clutches, reprogramming kit, shift kitted corvette valve body, corvette servo, new TC, the biggest auxillary cooler I could find, etc. After alot of screwing around and adjusting it's working great, but I haven't done any towing with it yet.
Some considerations for you whether you have it rebuilt or not:
Lock up convertor. The builder put a hydraulic lock up kit in mine and it works perfectly. There's no wires or switches to hook up. There are lots of links and info on this site to make your own electric lockup or you can get the parts from a donor vehicle. TV cable brackets. This is the part that fought me every step of the way. If you have a q-jet and the stock bracket it should be no big deal. I use a Holley carb and tried several brackets before I got it right. As far as the crossmember and driveshaft length I can't tell you anything about 2wd's.
With the overdrive my cruising rpm's are lower than what I like, because I have 3.08's with 33" tires. I'll be switching gears pretty soon though, and your C30 is probably geared lower. You'll like the lower first gear in the 700. I can't tell you about any mileage increase because I don't care enough to ever check it, and it's been winter since I got the trans put in so with warm up time and taking twice as long to cross town it wouldn't be accurate anyway.
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