Im getting ready to put my 305 in the car this weekend (finally!!!), and I needed bellhousing bolts, pressure plate bolts, and motor mount bolts... I couldnt find them anywhere including Summit and Jegs, so I went to the Pontiac dealer and found the sizes. So just to be a nice guy, and possibly help others with a swap, I thought Id list the bolt sizes for future reference to others:

Bellhousing bolts (T5): .375X16X1
Pressure Plate: M8X1.25X21.6
Motor Mount (the 2 long ones that bolt the engine to the mount): 7/16X14X5 1/8

The bellhousing and pressure plate bolts are standard bolts that you can get at a hardware store. The motor mount bolts seem to be a special automotive bolt and are available through the dealer for $2.71 a piece.

 

Pressure plate bolts are not standard bolts!!!! They are Grade 8, and they have a shoulder behind the threads to "dowel pin" the clutch to the flywheel in the exact right place. If you use regular bolts you will have a vibration problem and a safety risk that is IMHO unacceptable. Also, they are 3/8"-16 (at least on 2-piece RMS flywheels), and are available from Mr Gasket, ARP, and a host of other specialty mfrs.