A couple of things:
1) There are many starters that will work for SBC. The main differences are the flexplate/flywheel tooth count (153/168) and the bolt pattern (staggered/straight). From there, there are clearance issues, gear reduction, high torque, mini's, clockable, different bodies, noses, etc etc.
2) When you were attempting to troubleshoot the no crank condition, taking the valve cover off was unnecessary. For future reference:
Check battery voltage and connection integrity, check for B+ at the S terminal during cranking and if those exist and are good, use a breaker bar to ensure the engine is not seized. If it moves, replace the starter.
3) As far as whatever starter you put in there, although it might work for now, it would behoove you to have a new or remanufactured, correct starter installed in its place, if you are relying on your truck to get you around.