An inline 6 is inherently balanced, so unless you've done something oddball, like installed super-light pistons, etc., it won't be absolutely necessary. Oversize pistons are purposely lightened a little to match the weight of a standard-bore piston, so just overboring is not a compelling reason to do a balance job.
If you decide to go ahead with the balance job, make sure the machine shop includes everything that rotates in the job, flywheel, harmonic balancer, pressure plate, clutch, etc. I weight-match all the moving parts of a Chevy V-8 before sending the crankshaft to my balancer operator, down to 1/10 gram. (That's 1/10 the weight of a dollar bill, BTW). He computes the "bob weight" from the reciprocating and rotating weights of the piston/rod/wrist pin/ring assemblies. I-6 engines do not need bob weights during balancing, because the reciprocating and rotating weight is equally spaced 120 degrees apart. Your engine will probably run a little smoother if it's balanced, but it's not necessary for reliability.
Jerry
