Going from 228 gmc to 302 gmc is a fairly staight forward swap.
Some monkey wrenches to lookout for would be in the flywheels. A 228 will have a 4 bolt 6 volt flywheel. All 302 gmc engines have 6 bolt flywheels. Since 302s came out of either military vehicles or big trucks, the flywheels will likely be unsuitable for a 1/2 ton truck. The 6 bolt 6 volt flywheels from a 54-55 248 will work on a 302 and can be used with the original 6 volt stomp starter and 51 gmc bellhousing. Those flywheels are often hard to come by. The 56-59 270 gmc 6 bolt 12 volt flywheels are often easier to obtain. The 12 volt flywheel needs a 12 volt starter.
139 teeth = 6 volt flywheel. 168 teeth = 12 volt flywheel.
Next monkeywrench has to do with the front timing plate. Later civillian 302 motors used a different front motor mount. To bolt one of these civillian 302 motors in pre 1955.1 truck requires swapping out the front timing plate. The swap isn't hard but requires significant dissassembly of the motor. Here's a list of what needs to be removed to swap the timing plate.
Valve cover
Rocker assembly
Side Cover
Push rods
Lifters
Distributor
Fuel pump
Oil Pan
Harmonic Ballancer
Timing cover
Camshaft
Now you can get to the timing plate
Military 302 motors have their own unique challenges when installing in a civillian chassis. I haven't done a military 302 swap, so I am not versed in those challenges. More info can be found on the oldgmctrucks.com forum.
Next monkeywrench will be in the open drive rear end. Some welding will be required... unless you can find a 1955.1 rear axle and springs. The 1955.1 axle and springs will bolt into your truck and give you an open driveline.
