Need help with engine numbers

[Jimbotronics]

What's going on everyone? I know that the engine in my 71 has been changed out to a newer engine but I don't know how new - I've looked through the site about engine numbers and where they should be located but the only ones I can come up with is 435 on the front of the block and 638 on the back - does this tell you guys anything? I don't know much about engines, but I know that it has an electronic distributor (cap and rotor) instead of point and condensor.