Here is the way I see it. At a minimum you have risk of that piston continuing to fail and having the motor come apart. Any balance work you did is out the window anyway. I would start calling around to see if anyone has one, also if you know whose pistons they are you can contact the mfg and see if they know anyone selling by the each. I'd much rather have 8 pistons intact with one that is no longer balanced than one that is broken. If you had balance work done the rods and crank are still balanced so that is better than off the shelf. A buddy of mine has a '65 elcamino with a 565 bbc and I know he has ordered less than a complete set due to leaned out cylinders. I don't want to just say hey you need to spend all of this money, but I am very concerned about the reliability as it sits with that piston. I would think that you would need a new head gasket but I'm not positive. Definitely wait to hear from your machinist. I had a '67 charger that I rebuilt and installed a 383 into, I built the motor 3 months before I was able to start it for the first time and didn't know that all of the break in lube had come off of the cam and you needed to run the motor at first start up for 20 mins at 2000rpm to break in the cam. Well, the cam ground into the lifters and sent metal throughout the entire oiling system and I didn't get more than 50 miles out of it before I had to pull the motor, buy new cam and lifters, all new bearings and basically rebuild the motor a second time. I'd rather see you find a $500 motor and throw in your truck until you can afford to repair this motor the right way. Again I'll say wait until your machinist has a chance to look at it. Hoping for the best for you.