C4 trany problems

[73 XR-7]

i have bought a 1973 mercury cougar xr-7 in the fall and the guy who sold it to me told me the only problem it had was the engine was blown so i replaced the engine and i started it for the first time yesterday but when i put it into gear there was nothing at all no gear works! there is fluid in the trans when i put the engine it i bolted it up to the flywheel everything was already connected to trans and was working when he parked the car so i didnt bother touching any of that what could the problem be any input will be much apretiated

[MagicRat]

Did you actually observe the car moving with the old, 'blown' engine, or did you just take the seller's word that the trans was okay?

Are you SURE the flexplate (flywheel) is bolted to the converter?
(no joke..... I did this once when installing the engine. It took me about 5 minutes to figure out the converter was in place, but not bolted up.)

When you removed the old engine, did the converter fall off? When you put it back, did you rotate the converter a bit while pushing it onto the shaft? (this is what makes the converter engage the oil pump.)

If you are 100% sure the converter was installed properly and sure the trans fluid level is adequate, then remove the trans oil pan and change the fluid and filter. I have seen filter so badly plugged that the transmission would not engage any gear.