Yes I searched and read every one of the posts on the speedometer cable subject, but I need some clarification please. After my JDM engine swap, the speedo doesn't work and I don't know if the key didn't go into the slot on the trans or if the problem is at the speedometer. One of the posts seemed to indicate that the square end of the cable needs to "snap" into the back of the speedometer(?) Seems like it's simply a question of fitting a square peg in a square hole but after disconnecting the cable at the trans and sliding the cable back and forth in the cable sheath, I can't feel any positive engagement at the speedo. I tested to see if the square end made it in the square speedo hole by turning the keyed end with a drill motor and got nothing on the speedo. Do I need to take the gauge cluster out to get the square peg properly inserted? What's to stop it from becoming "uninserted" as I try to line up the keyed end with the slot in the trans?

Make sure the "key" that is on the end of the speedo cable goes into the speed pinion is in the slot thats probably your problem. I have never really had a problem getting the other end to go into the back of the cluster

If you haven't, take your gauge cluster out (sounds like you haven't). Getting the cable on the speedometer is a non-issue that way.