Its a 6.2L with a TH400 tranny, Im told the engine should go for 200,000 miles is this true? And Im told the th400 is a very well built transmission, hard to tear up. But how many miles will it last?? I appreciate whatever you have to say on this.

A 6.2 with a TH400 behind it will last, but only if it isn't interstate driven, because the TH400 is just a 3speed auto w/o a locking torque convertor. High RPM's increase the wear in any engine.

But if the engine is taken care of by the normal maintenance and you don't get it hot or run it low on oil, and don't let it eat dirt, it should last a long time.

The TH400 is known for it's torque handling capabilities, it lasts, but not any longer than any other auto tranny from 100k to about 150k, your mileage may vary.

I've been considering changing my barn yard 4 speed manual to a TH 400. Currently have 3:73 gearing front and back so wondered how well the TH400 matches up with this gearing. I usually have a lot of weight in the truck and will hopefully soon be picking up a trailer for hauling lumber around. With the manual and really only 3 gears to use on highway, I find that the ratio between 2nd and 3rd are too far apart. When I hit hills or even just getting up to speed, it revs quite high in 2nd then bogs in 3rd so could use better shifting ratios for sure.
BTW, I'm running an '88 6.2L 3500 light duty (no dually). Motor seems to have lots of pep but lose lots of momentum while shifting.
Thanks in advance