Just be careful to make sure you disconnect the Speedo cable before you yank the cluster out too far. Mine had a 2 part cable that I had to un-clip in the scuttle /wiper motor area. I didn’t know this and cracked my 10” section connector after pulling on the cluster without disconnecting it
Your ‘77 is older and may have a one piece cable with a metal thread on connector at the cluster. I have no experience with removing that setup, but others in this forum do.
All instrument clusters have three multi-terminal push-on connectors on the cluster back PC board. You just reach around and pull them off after removing the five Allen wrench screws.
MWB has the four larger LEDs for overall illumination or other sources have them, but be careful of length and diameter dimensions. If you want to replace the smaller bulbs with LEDs, you can find a multicolor pack of them on EBay. You must use the right color with LEDs or they’ll tend to look white-washed. Just be ready for a much brighter illumination of the cluster panel.
Also, make sure to plug in the three connectors and test out that the LEDs light up before buttoning everything up. This ensures they have the correct polarity and good connection in their sockets. If one or more doesn’t light up, it probably isn’t bad. If it doesn’t light up, remove the LED, expand the contact wires on the LED such that they would push against the socket metal tabs and/or rotate the LED and insert to reverse the polarity.
Good luck and don’t force things to much. It shouldn’t take much force to remove anything plastic or it may break these gentle old X’s.