This issue arises often.
In this thread (
HERE), Harry Moon confirms that the 6" fronts use a wheel cylinder that is the same dimensions as the large 1 1/12" rear, but only have a bore of 1 3/8". This "Special" cylinder (C118Q 2192) does not seem to be currently re-produced. All that are available are are the small front 1 3/8" cyl (Modern No W4571) for the 5" hub, and the large rear 1 1/2" cyl (Modern No W4511) used on all rear brakes.
You might consider using a W4511 rear on the front brakes, but you need a 1 3/8" cyl in the front to stop the front bakes locking up, however the W4571 although hydraulically correct size, is dimensionally too small for the width of the brake shoes. I think the bolting mount centres are also different too.
You need to start with the rear W4511, which is the correct size casting, but have it sleeved down to 1 3/8" bore. This reduction in bore size is probably beyond the size sleeve used by regular brake sleeving companies who do this work in brass or stainless steel. You might need to have the sleeving done in two steps, a coarse bushing to get closer to 1 3/8", then a a fine sleeve in brass or stainless to be the hydraulic finished bore. Assemble the cylinder with the 1 3/8" cups, pistons and spring from a W4571 rebuild kit (WK103), and the outer boots from the W4511.