343 crank with 6cylinder flywheel

Unless this is a really high powered engine I don't think the bolts not fitting tight in the holes would be an issue. Once the bolts are torqued down they are not the stress point. They would just be like a key in a shaft. The main holding power is where the the crank hub and flywheel join -- the clamping force is what's holding. The only time you stress the bolts to the side is if they happen to work loose and the flywheel actually moves. As long as they are just a loose fit and not really loose (like a 5/16" bolt in a 3/8" hole would be too loose, just an easy drop in wouldn't be) there should be no issues, but as Jim mentioned, a machine shop can come up with several ways to fix it, and make you a spacer if that's a concern. As long as the flywheel clears everything in the bell and the starter will engage the ring gear everything will be fine there.

My concern is if there is a balance weight on the flywheel for the 343 or not. I don't recall if a stock 343 flywheel has a balance weight, but I think all the Gen 2/3 V-8s have one. One of those will start with a flywheel without a balance weight (or the wrong balance weight) and run low rpm/idle with no issues. I've heard of people running 304 flywheels on a 360 and not having any noticeable vibration issues until 3000-4000 rpm -- broke engine in and didn't notice any issues until starting to drive it harder.  That doesn't mean there are no vibrations until 3000+ rpm, just that the engine mounts and harmonic balancer are absorbing lower speed vibrations -- will still reduce bearing life if run that way for a long period of time. If the crank has been internally balanced there should be holes drilled and filled in the counterweights. If not, and no signs of a weight having been added to the six cylinder flywheel, I'd either have balance checked by a shop (if the engine isn't assembled... I think you mentioned yours is) or get a 343 flywheel. Even if you have aftermarket pistons and such, the stock 343 flywheel should be close enough to run without any issues. Really only require balancing if running light weight rods and pistons -- that changes reciprocating weight a good bit. Stock rods and aftermarket replacement pistons shouldn't affect reciprocating weight very much, especially if the pistons were matched in weight. It's not a bad idea to balance, the better the balance the longer the engine should last, especially if you plan on turning it up a good bit often (like a drag race engine), but for a street engine having the crank balanced is just a personal choice. While theoretically it should last longer, there are other things that will give out before the bearings if it's well maintained.

Rick

That means you either have a 343 flywheel, or the six cylinder flywheel may have been balanced to the engine. I don't recall seeing balance holes in a six cylinder flywheel. If there are several balance holes around the face diameter that would be a good indicator that what you have has been balanced to your 343.

Thanks for those pics Rick, helps a lot!

Ah, balance holes!! Makes me wonder... do most six cylinder flywheels have a balance hole or two? I ran one a while in my car, and a Jeep J-10, but don't recall if it had any balance holes. Haven't had one apart in a while now...

Yes, 6cyl flywheels have balance holes as well.
Maybe someone can look up that part number - It is not listed on Eddy's site.

Looks like someone may have machined a counterbore in yours.
Take the pressure plate bolts out, lay it flat and measure the height to where it would mate up to the crank - I will measure mine to see if they are the same.
Also, sit the crank in the counterbore. There should be a slight press to a slip fit.
Looks like you may be good to go. You should still have a machinest machine the face and check for balance with the rotating assembly.