Oh, and after a lot of thought and research, I decided to go with YSS shock absorbers. The top mountings of these shocks are thinner than the original which turned out quite well for me.
Because the swinging arm that I am using is a K7 with drum brake fitted to an F1 frame. The swinging arm is not symmetrical, the left hand side of the swinging arm is further out from the centre of the wheel than the right hand side, (probably up to about 15mm more) this was I believe to accommodate the 630 chain. My rear wheel is laced centrally and is bushed to align with the centre of the frame. I also machined my hub to line up the sprocket and fit a 530 chain. Therefore the left hand shock needs to be spaced out away from the frame at its top mounting point, so that it is perpendicular to the ground and not canted in. Fortunately spacing it out 6mm appears to get it pretty good, I made two spacers, one to go inboard the LH shock and the other to go outboard on the RH shock. This would all have been a lot easier if I’d stuck with the F1 swinging arm, or got a K6 one that uses a 530 chain😀
Also my swinging arm has new lower mounting bushes fitted, but the lower fork of the shocks has about 1mm of play, so I shall be installing two 0.5mm shims each side of the bush so that the alloy fork legs are not compressed and distorted when I tighten the mounting bolt. Also, (wait for it😀) the original lower mounting bolts are too short for the new shocks by about 5mm or so, so I shall need longer ones.