Won't shift into gear

First fix the trans leak. Whenever engines are replaced by some times the torque converter slides forward then is not put back in then engine is bolted up and started which the torque converter slides out of oil pump then destroys it on start up of different engine if converter wasn't put back in towards trans then this is what happened.

If the torque converter was not in all the way and the transmission was bolted up then at least the front pump of the transmission will need to be replaced. Sorry, its a hard lesson and most mechanics learn it the hard way so don't feel bad. I speak from experience :)

See if you can get a used transmission.

Actually, second gear is the default for "limp mode". That allows you to drive slowly to a repair shop without needing a tow truck. As a general rule, if it goes to second gear as soon as you shift into "drive", it is a sensor or other electrical issue. If it starts out in first gear like normal, then bangs to second gear during or right after an up-shift, it is usually due to slippage in one of the clutch packs. That happens more commonly on high-mileage vehicles. Years ago we had a year or two of warning because we could feel the shifts become sloppy or soft. With your computer-controlled transmission, the computer knows how much wear has taken place in the clutch plates and it modifies shift timing to overcome that wear and provide continued crisp, solid shifts, ... Until the day comes when it can't update those shift schedules enough. That's when the slippage takes place with no warning and it goes to limp mode. The clutches don't actually wear out any faster. It's just that you don't having the warning it's happening.

Your mechanic will start by reading the stored diagnostic fault codes. That will tell him the reason it went to limp mode. He can also read the "clutch volume index", (CVI). That is a set of four numbers corresponding to the volume of fluid it takes to apply each clutch pack. An experienced mechanic will know how much life is left in each clutch based on those numbers.

Probably what ken just replied, the torque converter should have been put in trans instead of engine soyou probably damaged pump to where you have nothing moving in trans.

Make sure the converter is in all the way you'll know then bolt trans to engine after engine is installed bolt up converter. Converter should be about an inch away from engine at least after bolting up. Make sure the electrical connections will work as well.

Check with a junk yard they know better than us.

Sticking in second gear is "limp mode" that allows you to drive it slowly to a repair shop without needing a tow truck; definitely not at highway speed. In general, when it starts out in second gear and all the shift indicator lights are on, it's usually a sensor or other electrical problem. If it bangs back down to second gear during or right after an up-shift, it's more likely to be slippage in one of the clutch packs. Either way, there will be at least one diagnostic fault code stored in the Transmission Computer. Having those codes read is the place to start. You'll need a mechanic with a scanner that can access the Electronic Automatic Transmission Controller (EATX).

Scan the TCM for code/s could be a sticking shifting solenoid/s, you could also have it pressure tested to find out more-