Heres my situation ...
I blew my 3rd motor in my car. I cant afford to wait 2 months for a crate block. I found a local guy selling a rebuilt 420a complete motor bored .20 over. It has around 1000 miles on it. I have je pistons and eagle rods available. Would it be wise to pop the head off and swap the pistons and rods in with new bearing ?

do it.

Should I be worried about not having a balanced block ?

Take it to a machine shop and get it put together RIGHT, and balanced.

Not saying your not competant to do it yourself but, they assume the responsibility.

OR take the pistons & rods to a machine shop & have them balance them. Come home and assemble the motor yourself.

As long as your pistons are +.20mm, and you have new rings that go with them, assemble it yourself. There's not much to it, just replacing pistons, rings, rod & main bearins. All you have to do is hone the cylinder walls, and torque everything to spec (if you can read & comprehend, you can do this) you should be fine.

Our crank is internally balance already, and could probably spin 7500 comfortably, but the factory limit is 7250.

Dont the pistons and rods get balanced to the crank ? I guess you could balance a piston and rod by weight

Either way Im getting the motor on Saturday so I want to know if I should tear it apart and swap the pistons out. I can do a whole assembly from scratch I just run into a problem with money and a place to completely tear it down. I piston rod swap I can do at my apartment. I really need some input if this is a wise decision otherwise Ill just leave it as is as just continue boosting only 6 pounds

Typically, the machine shop balances each piston to be within .5 to 1 gram of each other (or whatever the weight/mass measurment they use), same with the rods. That way, each peice of metal the wiggle stick is slinging around is balanced.

If I were you, I'd do it, just remember to hone the cylinder walls.

So in theory I'd have a balanced block if I balance the pistons and rods. I'll deffinately hone the walls as well as change the bearings headgasket and throw in arp studs.
I just want to make sure im not making a mistake. Thanks for everyones help. Looks like Im picking up the motor this weekend and tearing it up. Any more input is appreciated.

you are really picky on getting this thing balanced. i bet 90% of the guys making good hp, just bought some pistons, and rods. got the block bored 20 over with relation to the pistons, and slapped it together. (as i did the same) and it still runs great.

Take apart the block, check the cylinder walls, check the deck for flatness, fix if necessary, set your ring gap and slap it together. Make sure the crank surfaces are clean and plastigauge it and run with it.

i got mine balanced but i guess it's not completely necessary, didn't cost too much to do either so i figured why not. But you'll probably be fine without it

I really appreciate everyones help. Made me feel a lot better. I love this site. Once i get my pistons rods bearings and rings im gonna weigh them on my scale to see what happens. If its far off Ill be back here looking for a local to give me a suggestion on a reputable shop.