Hey Hman, Sounds. about right. Don't have a '94 or a 5.7 but I had a '97 Sierra 1500 with the 5.7 where that canister probably would be easier to access? LOL.
Your first digit vin # is 1, 0r 2. #1 is built in Detroit, #2 is built in Canada. Dodge is the same.
Could just be a kinky vacuum line, bad solenoid or vacuum valve? I can look in my 1990 emissions manual, but all the TBI equipped vans with the SBC should have similar emissions systems for fuel tank venting. I have seen bad cannisters on GM vehicles, but something else is always wrong as well.
Van on! in Mexico City!!