273 and 318 doner parts for 340 Build

Do you think I should look at aftermarket connecting rods as insurance to prevent breakage?

I wouldn't be concerned too much about breakage... factory parts will hold up to quite some abuse when properly reconditioned and with decent bolts.
BUT, for the cost of having your 318 rods reconditioned you might be (almost) able to find a set of aftermarket rods... Scat comes to mind.