Sounds like my tranny is throwing around marbles

So I just put in a KA-T after blowing the pistons in my other motor, got the motor running good, go to put it into gear and it won’t it’s like pushing into a brick wall. If I start it up in gear the wheels start turning, then if I put the clutch in its just an assortment of metal on metal sounds. Got the driveshaft out and will continue to remove the tranny tomorrow but just wondering what I should e checking when it’s apart. Also the clutch pedal is very very stiff.

Tranny was working normally before switching over the engines? If so maybe the clutch disk was put on backwards?

Well when I pulled off the track once my motor started half exploding there was a noise in the tranny but after taking my pistons out in pieces I figured that was the noise, not the tranny. I hve a clutch masters dual friction disc so it’s very obvious what side is facin outwards although anythings possible. I will double check that for sure