Ok, I was thinking about this Later pontiac swap and I thought...
Perhaps you could make or use an adapter plate to bolt your bellhousing up to the pontiac motor.
Companies like this one
http://www.wilcap.com/webdoc8.html make adapters to use mate different transmissions to different engines.
http://www.jegs.com/i/JEGS/555/60170/10002/-1?parentProductId=751074 This might actually be the plate you need to put the chevy bell on the pontiac. You might also need a spacer plate to push the flywheel back the same distance.
