A 2 barrel 350 means the 400 was removed and replaced by an entire engine. I would have swapped the 4 barrel intake and heads just because the 4 barrel valves are bigger and the Quadrajet is a much better carb than the 2GC. That said, the 2GC carb is what they used on basic cars when the rear end has low highway gears. It is good for what it does and is relatively trouble free. Some of my best cars used it. A 1968 400 was a pretty high compression engine. It may have not survived the switch to unleaded gas and/or been replaced with a low-octane-gas-friendly low compression 350. Any 400 you consider should be low compression too. My gas tank is nasty too. In the old days I would just take it to a radiator shop and have it boiled out. I don't even know if such shops are still around. There are two options: get a new one, or clean it. You drop it down and remove the gas gauge assembly, empty anything still inside, and pour solvent into it. #2 heating oil [diesel] is good because it won't explode. Then let it sit for days, slosh it around, pour it out, keep doing that until the tank is clean inside. If you try and just pour fresh gas in as it is, expect it to be filthy inside. As gasoline evaporates it leaves gummy deposits that will cause mischief later on. In that case I would remove the gas line hose where it makes the jump from the frame to the fuel pump. Install a big in-line fuel filter in that line to catch all the crud. After a few tanks of gas have been used you can remove the filter and toss it. It is always a good idea to keep an in-line filter at that spot. This way it catches any debris that otherwise would clog the filter in the carb. No pics. My car is stripped and has a busted windshield, so, ugly is the word. I am replacing rusted floorboards and doing improvements so lots of work in progress. One last thing: don't be too quick to tear into that engine. I made that expensive mistake on my first 350. If the oil pressure is good, compression is good, no expensive noises, no obvious leaks, then all you need to do is get it running and then evaluate it. It may be fine and just need some new gaskets and fresh paint to look great. Find out what year it is and check the engine code on the front of the block just under the pass side head. That will tell you what it is and how it was used.
