Clutch, Flywheel - 305 to 350 ? - Part #'s?

[PHOENIX]

1983 1/2ton 2wd with 4 speed manual (465)
original with 305 now has 350.

Ok guys clutch started slipping about 2-weeks ago.
Pulled the trans replaced clutch with beck/arnley 11".
Started the truck had rubbing noise, pretty loud all the time.
Not sure what caused it pulled the trans again.
Couldnt find anything that could have caused it.
Going to change to performance clutch this time while everything is apart.

Been checking website and catalogs.
305 came with 11" clutch (which I have now with my 350)
350 should have 12" clutch (which is the only performance clutches I can find listed for my truck)

The flywheel and bell housing seems to be the same.
Can someone verify that the flywheel and bellhousing for a 305 and 350 are the same (not will they work, but are they the same, part #'s maybe)?

Here is the issue, when I measure the flywheel to see if a 12" disc will work, its too close to the pressure plate mounting holes. I think its too close for a 12" disc to be in there.
SO IS A 12" CLUTCH DISC REALLY 12" OR IS IT LIKE 11-1/2" OR 11-3/4" ?

Going to try and contact someone at either jegs & summit to see if they can help. I am also going to try and call a couple of the clutch mfg's. Was hoping you guys may be able to help some.
Need to order the parts asap so I can get this finished next weekend.
Thanks