You do have some options here, first being forget about whatever bellhousing and clutch came with the 302, it's way too heavy for use in a 1/2 ton pickup. Use the stock 228 1/2 ton bellhousing which **should** mate up OK with the 302 block but follow the shop manual procedure to dial indicate the bell housing to crank spec, adjust dowel pins if grossly out of alignment but chances are its OK. Your local clutch shop can reccommend a diaphragm clutch that will bolt up to your 302 flywheel (but a 270 wheel is lighter, same pattern). A B&B style (3 finger) clutch to match the GMC bolt pattern won't fit in that bellhousing, at least not that I could find.
Another option would be to obtain a SBC V8 nodular iron flywheel and have the flange and bolt pattern machined to fit the GMC crank. Then you have an unlimited variety of OEM and aftermarket clutches to choose from, cheap and plentiful. Ring gears are same as the GMC (12V) flywheel so starters should match. All of these clutches should fit the 1/2 ton bellhousing with no problems.
