55 and up 235 and 261 use the same bolts,try this place from the links right here on Stovebolt NOS parts they show 12 new rod bolts in stock#3836670 for 55 and up 235-261.i have bought from them several times, slow but reliable service.
The stock 3/8 rod bolts are quite beefy for the power output,no need to change them unless you suspect they have been over torqued in the past