Do note that the real ones had the body of the muffler wrapped in asbestos rope, with a metal heat shield over that:
View attachment 194238
The kit parts don't seem to replicate the heat shield, though. The mufflers are held to the hull by U-shaped rods that go around the body of the muffler itself, as shown on the parts you have:
View attachment 194240
The heat shield sits "outside" of the brackets, but I'm not sure how it was held on:
View attachment 194239
The first and third photos are of the unrestored Whippet in Brussels that still has the heat shields, the second is of the one at Bovington that doesn't, and the rope on the latter looks to me to be a (polyester?) replacement.