I just had a heart heavy barrel installed on a Mcmillan A5 and you can barly slide a slip of paper down the barrel channel, is this enough?

You don't need a whole lot of gap there with a synthetic stock...and if it's too open you just invite debris to get in there.

I have a couple of McMillan stocked rifles (done by McMillan) and there isn't a large gap between the barrel and the stock. I can slide a dollar bill in there and run it back and forth.

When I glass bed an action I like to bed the entire barrel channel and in doing this I wrap two layers of electrical plastic tape around the barrel so that after the glass has hardened and I remove the barreled action from the stock and then remove the tape there's about .02 gap all over between the stock and barrel.

The glass in the barrel channel is a big aid in resisting any warpage in the wood and the gap is small enough that it's rarely noticed.