68 galaxie mystery

Glad to have found a forum such as this, less happy about why i had to set out to find a place to gather information however.

Recently I decided to take my 66 plymouth fury II off of the road for a long over do rebuild. It didn't take too long for me to severely miss driving an older car so I acquired a 68 galaxie(factory 302 and 3spd auto. fmx i believe) to substitute as daily driver. It has been giving me hell from day 1, well technically day 2. Since i'm new i'll share a picture before i really get into my question.

The car runs pretty fantastic as of right now, until we have a day that requires quite a bit of driving. Anything over ~20 miles and the car goes from running fine to just falling flat on it's face. Feels kind of like timing or just misfiring, starts out fairly sporactic but then becomes undrivable within a few miles of driving in town. If I let it sit for 30 minutes or so the game starts all over and I can drive fine again. I'm sorry for the length of this post so far but I'm frustrated and at a loss. I'm not a mechanical genius but i've been able to keep cars on the road pretty well up until now. Can anyone either point me towards literature to help me understand this motor better or possibly dealt with something similar and could simply give the opinion?

While tracking this problem down I have done/replaced the following (some were just because it was cheap enough to play on the safe side)

New timing chain and obviously dailed timing in after that job. When the problem first presented I assumed the car running rough was because it had jumped a tooth/teeth. I was wrong but the chain had much too much slack so the job wasn't entirely for nothing at least.

New dizzy and cap

"New" edelbrock carb to replace factory autolite 2100

New coil

oil and trans fluid change along with filters.

plugs, wires and fuel pump were replaced days before i took ownership

probably missing a minor thing or two but that about covers it.

again i apologize, hopefully I find a few questions in the forum that i'm able to contribute to instead being just another random nobody with nothing to contribute.

and just for fun. a picture of the plymouth before she was ripped into a million pieces.

Did you change the fuel filter?

I'd check the plugs to get a sense of the running condition.

I'd check the plugs to get a sense of the running condition and check compression that time.

Sounds like it is vapor locking. Check your fuel lines location to be sure it does not run too close to the motor or exhaust manifold causing the fuel to vaporize in the line. The longer you drive, the hotter the fuel becomes, thus it will run worse until you stop and let it cool. Very common on older vehicles.

The pick-up tube in the fuel tank has a strainer on it. Rust and other particles in the tank will be sucked up against the pickup until it is clogged and not enough fuel can flow. The car sits for 30 minutes and it all falls off and the car runs great for another 15-20 miles. Drop the tank, clean and seal it and get rid of the strainer on the pickup tube. Use an inline filter before the fuel pump to catch any remaining trash in the tank.