I was just ready Now and Forever for the 5th or 6th time (probably more) and each time I cannot get over the parallels between Summerton and the town of Spectre in the Novel/Film Big Fish. They both follow almost the exact same premise of two protagonists which have stumbled upon or are having a premature view/visit to the afterlife. Although there are obvious differences (people do not age or age at a snail pace in Summerton, for example) the similarities are numerous.

The fact that there are characters which have already passed in the story (Hepburn/Nef) makes it clear that this is a representation of the afterlife which Cardiff has stumbled upon prematurely and, like Bloom in Big Fish, eventually leaves/escapes so that they may find it later at their correct time and place.

My question is, although this novella was not published in this form prior to Now and Forever (so I believe), had it been previously published in another form such as a short story? The similarities between the two stories are too close to be mere coincidence so I'm curious if it is possible that Bradbury inspired Wallace's novel or vice versa.

Not published, no, but SOMEWHERE A BAND IS PLAYING was developed as a screen treatment decades before Bradbury finalised it as a novella and put it into print. I haven't read BIG FISH, so can't comment on the similarities, but I don't imagine there was any direct influence.

It would be interesting to find out if Wallace knows Bradbury's work, though.