Here it is for those of you who dont want to access the gmc site.
No difference internaly.
External stuff is:
-Large casting lump above starter. Can be ground off.
-Side mounts off side of block. These cover the spot where the mechanical fuel pump would mount on the "civilian" engines. The military engines use an electric fuel pump in the tank.
-Real deep oil pan. Uses different oil pump cover so that the pickup can reach way down in the pan. A 228 pump cover and oil pan will fit right on.
-Oil dipstick on the side of the oil pan. Cast boss for "civilian" dipstick on the block is there; it just has to be drilled.
-Long pulley on water pump. As well as the dual pulley on the crank.
There are other small differences, but these are the big ones.
Here is some info from another post I made a long time ago:
-The military 302 has side arm mounts on the front of the block held on with studs. These studs will work to hold the generator mount braket on the left side the others can be removed and bolts sealed in their place.
-You can either get a civilian timing cover plate from ANY other GMC 6cyl, Chevy 235/261 or drill two hole and use bolt as per original.
-The oval hole for the fuel pump has not been cut out and an electric pump is used. However, the camshaft does still have the lobe for the pump and the block has the provisions to cut out the hole.
-There is a mounting boss cast on the block in the way of where the civilian starter will mount and this needs to be ground off flush with the block surface. No one really knows why this was cast on the block but no harm is done by removing it. The starter for the military hydramatics used in these trucks mounts lower on the bellhousing along side the oil pan and the boss is not an issue.
-On the rear of the engine a different bellhousing is needed as well as a 6 bolt flywheel. These will bolt right on to the block and flywheel respectively. The crank is drilled for a six bolt pattern and the same "rare" 6 special bolts and lock plates used for the hydramatic flexplate will work to secure the flywheel.
-The oil pan for the military configuration has an extra capacity extention added and extra tubing inside to get the oil pickup screen all the way to the bottom. An oil pan off ANY other GMC 6 cyl will be needed as well as the oil pump cover from ANY other GMC 6 cyl. The only thing to be careful about is that the pump cover used is matched to the capacity of the oil pan used. Some covers allow the pickup to sit farther down for the deeper pans. The pickup screen can be used from the military setup.
-A distributor from ANY other Chevy or GMC inline can be used. A hole will need to be drilled to hold down the vacuum advance.
The oil dip stick is mounted on the side of the oil pan in the militay setup. There is a boss cast on the block in the same location as on the civilian engines but it is not drilled. The hole needs to be drilled with a step in it. I can't remember the correct dimensions but e-mail me if needed and I will dig them up. The military dipstick tube needs to be cut and put into the drilled hole. The military dipstick can also be used. Once the engine is ready for fresh oil, put in 8 quarts and cut dipstick tube so that the dipstick indicates the correct level.
-The water pump pulley will need to be changed. A water pump from ANY other GMC 6 cyl will work as a replacement. New water pumps can be bought at local auto parts store and the Master brand comes with 3/8", 5/8" or 7/8" pulleys installed. The nice heavy duty 5 blade cooling fan can also be used on civilian GMC inlines and will bolt right to civilian water pump pulleys. The blades may need to be notched for clearance with the crank damper.
-The military crankshaft pulley dampener is the 7/8" two groove type and can be replaced with any dampener from ANY other GMC 6 cyl or even a 235 or 261.
-There is a cooling bypass line that is routed along the valve cover. This can be removed and the holes plugged. The military 302 also runs a compressor with extra cooling lines and oil lines going to the compressor these can be removed and the holes in the block plugged. Basically just configure the plumbing like any other civilian GMC inline and plug any holes left over.
-The military carb used is a sealed Holley two barrel. If the stock intake manifold is used, any 2 barrel downdraft carb will bolt right on. The exhaust manifolds are the same on military and civilian 302's.