Dad's 1966 Dart

The motor that's in there is so far gone that the crank moves back and forth. In willin to bet it will need machine work. So, for someone in your shoes bro I would recommend a cheap pick a part motor out of a wrecked truck to work with. A rebuil kit (bearing kit) can typically be had for around $200+ pick a part motor $200 so rani hit it on the head with about $600. I've seen people selling running motors that are still in vehicle (can hear run) for about the same price. As far as that goes, it's up to what you want to do.

as for messing it up: your putting you love and effort in to it for him. Your not gonna mess up anything in his eyes :)