Oh, I get it now. Yeah front and back movement is normal, I was thinking for some reason that is was lateral movement perpendicular to the driveshaft. Sorry.

I say rebuild for the following reasons
1. adds character
2. then you know you have a solid engine
3. kinda fun (in a greasy sorta way)
4. you can customize it (torque cam, compression ratio, etc)

My grandpa used to be a mechanic back in the day. He said with the old rings that used to be used, engines would have to be rebuilt at 30-40k. He said he would do the rebuild with the engine in the car (not recommended, I mention this as trivia only). He wouldn't change the cam (they were all pushrod engines back then) but he would pull the pan, the head, pull the crank, knock the pistons out, slap new whatever in, and put it back together. I have never seen this done, but according to gpa, it was fairly common practice back in the day.

best of luck

-frozen