Jimbo
The design is basically unchanged from Plunger/Semi Unit to the end of S/A. That centre bolt is only slightly inclined to the rear, so much so it is often overlooked, but for true perfection it's this one first out, last in. Also there are variations in the lengths of the head bolts.
Centre and Front 1 1/2" QTY 3
Front Outer 2 11/16" QTY 2
Rear Outer 2 3/8" QTY 2
Rear 2 11/16" QTY 2 All bolts 3/8"
Source: BSA Service Sheet 204 These variations accommodate the different positions within the cylinder head casting, but lengths are useful to establish you have the correct size bolts, not some spurious assortment. All have washers.
You can also try cleaning up the top of the valve stems with an oilstone. This area is normally hardened, and the hardness should extend deep enough to allow a light refinish of the top surface. Be careful, keep it square with the stem. If you find the material relatively soft, I would suspect the quality of the valve. A good, old genuine item is to be preferred over a modern repro, as you have a greater confidence that the metallurgy of the old component is at least up to the required standard as originally specified. We like to think newer is better, but it's not always so.
Swarfy.