You asked this question over at stovebolt too. Those rods would have to be really really beat up to not be rebuildable. The rods need to be magnafluxed to make sure they are not cracked. Have them checked for straightness. Then get some big block Chevy rod bolts. New Rod bolts for a 261 are like non-existent. Have the caps machined down, install the rod bolts. Resize the big end back to specification. Contour the rod bolts so they are shaped at the head like a 261 rod bolt (make a template if you want so you can check the work.). When the machine work is all finished, balance the set.
The GMC rod conversion is more work then most know. Not many have actually done the conversion. I have. The rods need narrowing in addition to a good rebuild. Rod bolts are the same issue, they need massaging. The gmc bearings need to be chamfered or in my case, had to be narrowed. The cam has to be clearanced between the lobes to clear the rod bolts and if you do it after the cam is final ground, you have to send it back because all that hand work bends and twists the can and it has to be refinished. This swap does need custom pistons. My motor is pretty modified and will get to 6000 rpm in a snap. That is why I did the gmc rod conversion. Unless you are planning on really getting on your motor, the 261 rods will be fine, at least in my humble opinion.
The thread for the conversion is mine. When you read through it you will see what a lot of work it was.
https://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=90677&page=1Go back to your machinest and ask him what it is about those rods he is concerned about. A couple of builds on the rods should not be that much of an issue for a good machinest.
This is my son driving my 57. It’s a street motor so not a race motor and he was driving it (and going down a race track) in dad’s car for the very first time so dad (me) told him to take it easy. Like I said, unless you intend to really hammer your motor, use the 261 rods.
https://youtu.be/lMVz66tyrKQ
