The plugs are numbered in accordance with the cylinder numbers.
"Now you've got to find out why and where the coolant is coming from." - Do you think pulling the head will answer that vexing question?
Because they were just photo'd in a straight line group in numbered order, I wasn't sure if they were numbered in accordance to the Chevy, with 1, 3, 5, 7 on the driver side and 2, 4, 6, 8 on the passenger side.....
Or if they were done Ford fashion, 1, 2, 3, 4 on passenger side, 5, 6, 7, 8 on drivers side.
(Because that puts #5 in two different locations)
I would do a leakdown test in that cylinder before pulling the head so that you might discover if the leak is head gasket or cylinder wall/head crack versus it coming from the intake or exhaust port leak (intake gasket leak or exhaust port crack).
A note....the 400 deck surface is known for going out of flat more than any other small block (large bore means less deck material), so if the block wasn't deck true'd at rebuild it could be a head gasket issue....but you have to test your way to that conclusion logically, not just assume it "has to be the head gasket".