What can you tell me about my truck and the 261 I6 in it?

[Thayer03]

I just bought a 61 Chev 3/4 ton pick up.
I checked the casting number on inliners.org and it tells me that I have a 261 in it. Are these engines worth keeping or rebuilding? It has a 4 speed tranny behind it. I have heard that these are extremely strong transmissions, it this true?
I also found a tag on the front of the drivers door jam with the following numbers. Could anyone tell me what they mean or if they are of signifigance?

Model- C3604
Serial- 1C3604613783B
Engine- C588849

The casting number on the bottom of the block on pass side for engine is 3769717 The is the letter M in a square below this number. What does it mean?

Also I found a number on the rear end will that tell me what kinda rearend it is? 3766686

Any help would be great! Thanks!

Randy Thayer