This sounds way too familiar! I have a '40 1/2t with a '58 235 that this summer behaves just like this, fuel starvation after running awhile. I have put at least 8000 miles on her over 2 Years without changing anything but gas and oil.Vapor lock was suggested and I tried an old "fix" of clamping wooden clothes pins to the fuel line(?), and fabbing a new bracket to hold the fuel line farther away from the front or the block. No help! Crud in the tank doesn't seem probable due to no crud visible in the filter and the fact that sometimes I can drive it 30+ miles at a shot without a wimper or hiccup. I will now try replacing fuel lines since the problem seems to be heat related somewhat(100+ degree days might soften an old rubber line). Thanks, I hope it helps.