LA block hunt/search maybe shift to piston hunt
It's no problem to put in a 273/318 crank with any 340 pistons, standard or lightweight, if you use SCAT I-beam rods, and you will be able to balance it by taking weight off of the crank. A lot cheaper than a stoker kit.
And if you buy the SCAT rods, they are balanced to each other and have the big end and small ends weights marked right on the box; those are the hardest numbers to weigh yourself so that makes the rest easy-peasy. Buy KB or SRP or SpeedPro pistons which are well balanced to each other, and use those piston and pin weights and the rod end weights and a few pretty standard weights to compute up the bobweight yourself. Hand that number and the 272/318 crank to the machinist and tell him to balance to that bobweight. Cheep; cost me under $100 to do this.
Let me know if you need help with the bobweight computation, but you can get the info and the formulas right off the Eagle website. Use 59 grams for 340 ring pack weights, 4 grams for oil, 4 gram for locks, and 44 grams for a rod bearing set. Or whatever you weigh to be most accurate. Then use the piston, pin, and rod big end and small end weights that you weigh or get from the date sheets. Checking the weights of as many parts as you can helps, but you'll be no worse than factory balance if you use the numbers above.
No need to be scared of this process if you can add and multiply; it is just what the machinist will do. Especially for a street motor.