Adding a power brake booster.

Back in the days of doing forklift wheels there were 2 descriptions for wheels to suit the Pommy stuff and it was either a 6 stud or 8 stud British pattern.
15" rims maybe only 5.50" wide, 16" should be 6.0" wide.
Try a Google search for the details of 6 stud British and see what turns up.

Bedford, six 18mm studs on a 205mm PCD (pitch circle diameter)

Tyre 7.50 x 16

My Terrier has the same 6 stud /16" wheels as a Bedford with duals ,the wheels are interchangeable .....the studs on the terrier ,which are also the same as the Austin 245F s so popular on this forum ,are 3/4 BSF LH&RH.

Imperial change over from metric should be 6 studs - 3/4" diameter on a PCD of 8 1/8"