It's a pretty tight fit on the 351, but there's a little give to the grommet, so pull it out until it touches the intake, then pull/push it toward the back of the engine while still pulling it out, and it should pop out. Sometimes it helps to pull the vacuum hose first. Bottom line is, you just have to force it a bit.
I just changed mine on my 302, all I have back there is a hose, just get your hand under there, pull back and forth a bit, make sure your truck is cold, tricky part is just getting off the hose and back on. Its not bad though.