I have a 261 truck engine and flywheel that had an 11 inch clutch mounted on it. I.m putting a car bell hsg on the engine and don.t think an 11" clutch will fit in. Question, can I drill new mounting holes in the flywheel to accomodate a 9" clutch. Concerns - NC machine work should be able to locate the new holes near perfectly about centerline of flywheel??? The new holes will be in the web (thinner part of the flywheel
)but the thread engagement depth looks about the same as the 11" holes because they are countersunk, the 9" holes will have no countersink. Spin balancing - is the flywheel balanced with the engine or clutch attached, or is it needed as this engine is not hot rod bound. Thanks in advance for your response.

Where your going with Engine / Bell housing set-up is kind of a mystery but drilling the hole pattern on your flywheel is fairly simple...and easy if you at least have access to a milling machine w/digital readout and a copy of the Machinery's Handbook . Using a dial indicator to determine the center...zero both x & y axis...look in your Handbook under circular hole patterns and you'll find a simple straight forward formula for the desired number of holes at the proper diameter...don't sweat it , your pressure plate holes have a little play . Of course you'll no longer have any centering pins , if you ever did . I occasionally use a dial indicator on a magnetic base ( placed on the engine ) if the pressure plate has a machined edge to run the indicator on , to center the plate a little more precisely .
I assume this is the flywheel that came with this engine so as far as balancing is concerned I wouldn't worry...
So what are you up to...? If it's not a secret...?

Is the 216 flywheel the same. I have a spare 216 flywheel. I think the holes need the countersink. Clutch bolts are bodybound.

Please don't take this as a smart-aleck answer but why don't you see if it will fit the 216 housing or whatever before worrying about drilling? It may fit. Good luck with it.
Charlie