Welcome to FTE and the Ga chapter. I would honestly rather have the 300 inline six in a truck than most of the v8 options. The peak torque is in the low rpms where it should be. If you can find a donor truck setup how you want it it would be easier. It would be helpful to know what v8 you plan to use and what your primary purpose for the truck is. Leisure, hotrodding or work.

For the engine swap you need a different radiator on inline 6s the upper and lower hose hook to the same side, on v8s they hook to opposite sides. As far as 302s and 351Ws go the radiator offerings the parts stores sale are simply wrong. You have the choice between a radiator that will not reach the mounting points (Can be remedied by making an adapter). The radiator that will bolt up has the larger hose connectors for the FE engine family (360,390 etc.) This can also be remedied. They make rubber adapters you can put on the water pump and thermostat housing to go to the larger FE hose. I think I have some if you can't find them. If you are lucky enough to find a good radiator for a 302 in the scrapyard make sure its got the hookup for the tranny lines. A radiator from a 70-79with 302 should bolt up. If you go with the Fe the radiator is easy to obtain but other goodies are not as easy to obtain including the auto trans. 460s are a whole different ball game I will not get into them in this post because I have not done that swap. I can throw you what little I do know if you ask.

Next thing you are going to have to obtain is engine perches for whatever engine you are using. Scrapyard hunting time.

Your three speed trans is shorter than an auto trans so you will have to find the right driveshaft in the scrapyard or take yours to a driveshaft shop and get it shortened. A shop near me shortens them for around 50 bucks but it varies. You also need the tranny crossmember for the automatic. In a truck a C6 trans is the better choice in automatics if you are going to use it as a truck. You can go with an AOD but you have to adjust the TV cable perfect or you will burn it up in a few miles. Sorry I do not know the adjustment procedures.

If you want shifter for automatic on the column you will be hunting alot for a good column for the automatic. You may get lucky and find one. It would be easier to go with a floor shifter. They can be obtained from the parts stores or you can get one off various cars withe the same type tranny you are going to. As far as the clutch pedal you can take it off or let it hang.

I may be missing a few things someone else may elaborate on them. I will also post if and as I think of more. Good luck on your swap. If you have any more questions we will be happy to help. I know from my long post it sounds difficult but it really is not.