New factory auto tranny purchased in 2003

So here's my question. I bought a bran new in the crate tranny from Mitsubishi in 03 built a high performance motor at the same time. Only like 170 hp ish. My dad killed it though. Moving on. The tranny worked great but 200-300 miles later the motor took a shat. Thanks dad. Tranny sat umfrom 2004-2012 without being used. I Slapped a jy motor in last month and it slips like a dirty hooker. I drained what was in the pan. Put new in. Would sitting for 6-8 years not being ran do the tranny in? Or is there something I need to do?

Are you using the tourqe converter for new tranny or junk yard one and was it filled.

Pour in a little Seafoam or Trans-x.

the tranny sitting for years will only affect it if it was exposed to moister and bone dry inside. even if you drained it before storage , there would still be a coating on the gears to protect the metal once you put it in a dry secure location.

Are you using a new flywheel or junkyard flywheel?

Flywheel is old off Jy engine torque converter was new with tranny and no I didn't drain it. Could one of the plugs on the tranny be bad causing it not to shift. Tempted to go pick up this manual tranny zanier an hour from
Me with 60k miles on it fòr $125 guy has a huge jy and just wants I get rid of it. I guess I need to check all the harnesses? It shifts gears if I baby it. But it just slips a little sometimes or is very hesitant to shift. I've had it up to 50-55 and it seems like its shifted into all gears properly just not very well. Has no kick down either .

Oh was sitting inside a garage as well not
Outside or anything. Actually was still in the car from when the other engine took a shat I actually do have a quart of trans x in the garage. Should I use it?

The auto trannies are know for taking a crap at the stock power lever let alone one putting out more. I killed 4 rebuilt auto trannies in 1 summer. I would find a used tranny lower in mileage and try it and hopefully u like driving stick cuz that is in the foreseeable future you you. The swap to 5 speed was the best thing I ever did to this car, yes even better than turbo.

Screw it. Down with the auto and off I go to get that manual up north of me this week. Wow posting messages on my phone makes me sound like I'm foreign or just have really bad grammar. That last one was horrible.

Should I have the manual rebuilt to hold some power? Or just throw in a high perf clutch and call it a go?

If the tranny is in good shape then there is no reason to tear it apart. I have a stock clutch holding 12 psi like a champ so don't go crazy with the clutch as all a heavy clutch will do is wear out the thrust bearing on the crank. If you plan on big power find another tranny later on to rebuild.