You mean, didn't get caught. There's a difference.
They'd have to have kept those crimes to extremely petty ones at the most. Even though the 1960's didn't have facial recognition, the TSA (for what that's worth), instant background checks, widespread Social Security Number checking mechanisms, or any of the stuff we have today? They definitely had fingerprinting, and at least some semblance of a national fingerprint database of sorts to check against (the FBI would have had these guys' fingerprints after the break.)
They could have eventually slipped through the cracks even if they re-offended (e.g. it wasn't uncommon for, say, truck drivers to have multiple drivers' licenses from multiple states), but any crime beyond a misdemeanor would have had the local PD looking at some stranger (stranger in their town that is) and doing at least a cursory check, if only to build a rap sheet for the prosecution.
IMHO, if they made it, they likely hoofed it to Canada or Mexico (or perhaps further South) and built an assumed identity from which to live out the rest of their lives in as obscure a manner as possible. Over time, that new identity would become reinforced.
It wouldn't be the first time either... I recall a few instances in the '80s and even the '90s where some schlub or other escaped prison in that era (or before), got himself a new identity, and decades later did something stupid (IIRC, in one case the dumbass ran for a local public office, and a local reporter researching his background found the inconsistencies).