My 440 was originally out of a motor home. It is a 1974 with a forged crank. It had six pack rods. I took it .030 and used cast pistons with LY rods. Would you have gotten the rotating assembly balanced? I am thinking of taking it .040 over with forged pistons. Would you have it balanced then?

Any time you deviate from a stock component in the reciprocating assembly that doesn't have the OEM weight, you should rebalance.

I'd balance even a stock rebuild, but I can't leave anything alone...

It really all depends on how you are going to use the engine. I have swapped out factory cast pistons for hypertic or forged pistons before and didn't rebalance. I have also broken a motor and needed to be racing the following week. At the time my dad had a box full of stock rods and factory cast pistons. We just mixed and match a set to get an engine together. I mainly drag race so I spin my engines up to 6000 or so. Now these engines aren't are smooth as they could be, but not sure they were any worse than some of the OEM balance jobs. I once used a shop to balance my assembly and it wasn't any smoother than some of the motors I slapped together. The shop I use now the guy is super picky about the balance job and those motors are SMOOTH. I'll balance all of my stuff from now on, just mainly because it's hard to find good unabused OEM stuff to slap together.

Balance it.

If the current engine doesn’t have any vibration issues then compare the new piston against the old.
With a gram scale, assuming the new pistons are heavier, you can match the the new pistons to the old.

No need to balance the whole assembly then.

Are you feeling really lucky or hoping to get lucky
Do it correctly once or maybe, probably, have to do it over later
I always cost more later to do it over

Can you destroy an engine without balancing?

Depends how bad it is out, and long you run it and at what engine speed.

I would think most people would not drive it long enough if it vibrated badly.

But then again.........

A friend bought a 69 charger 4 speed car that had a cast crank externally motor in it when he bought it. It vibrated. The car had the original neutral balance flywheel. It beat the bearing out of the hub for the pilot bearing and may have damaged the transmission. The rear main leaked as well. Suprisingly, the main bearings looked ok.

I don't know how much balancing would cost where you are, but I've never seen a price so out of line that I wouldn't do it. My 440 was balanced with the damper and flywheel, and it runs incredibly smooth, I thought it was money well spent.

If some how the price is outrages or this is an engine that "doesn't need it" I'd at least do what Transman suggests

Some people over balance, add extra bob weight. Some under balance , remove bob weight. The factory tolerance cylinder to cylinder is not spot on. How much can you get away with? Obliviously the factory specs work. Engines run a long time being untouched. Now lets say the new pistons are close or slightly under in weight. Is it a big deal being under balanced, probably not. Where you get into trouble is like a cast crank scenario. The bob weight is far greater than the crank counterweight mass. In this case extra counter weight mass must come from the flex plate, converter, flywheel, balancer. Is balancing a bad thing? No. Is it absolutely necessary when weights are close in a street application? In my opinion, no.
Doug

What I think you have to keep in mind that the magnitude of any unbalance increase as the square of the speed. Double the speed, four times the force due to unbalance. So if you have an engine that never gets over 4000 rpm, it can tolerate considerably more unbalance than an engine turning 8000 rpm.