273 and 318 doner parts for 340 Build

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.

But the aftermarket rods do not have oil squirt holes, where the factory rods do... I prefer to spend the extra $40 and get new bolts installed and recondition the factory rods....

I like the oil squirt holes to help keep the bores oiled...