In my MGA world, I would balance the crankshaft. I would also balance the flywheel and the clutch pressure plate as a unit. Each would cost about $75 to $100 U.S.
I would try calling around to some other machine shops or perhaps press your shop to just balance the crankshaft. I've not been aware that the rod and piston assemblies are necessary to balance a crankshaft.
However, I would also "try" to balance the conrods, the pistons and the wrist pins, to get them as close as possible to the same weight, even if I did this myself.
A set of pistons usually comes with relatively the same weights, maybe a couple of grams off one way or the other. They can be balanced to each other by drilling material off from the underside.
Wrist pins can be balanced to each other by grinding material off the ends of the wrist pins.
Con rods are the hardest. The big ends and the small ends are supposed to be balanced independently. Making a jig to do this is a trick. But you can certainly remove material from the conrod to lighten them if you need to.
I would consider taking each part and weighing it (I make a chart), then mix and match (to the extent possible) to get each assembly to the same weight. While you may be unable to swap the #1 conrod with the #3 conrod, you may be able to swap the pistons and/or wrist pins to get the units closer to the same weights. In other words, put the lightest piston on the heaviest conrod, etc. Once you have all of the weights charted, it's pretty easy to see what combinations yield the best results.
Then be sure to mark the parts so you can always identify where they are supposed to go when assembled.
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