Where would be a good starting point for needle and springs for a pair of 1 3/4 su's on a 73mm stroker 286 kent cam and a GR mild head

Not sure about twins, but with a single HS6 on a GR manifold, an OA6 needle and red spring worked for me.
This was on the 1412 stroker, dyno figures are in my sig below.

These needles are also used on a Healey 3000, so do work with multiple carbs.

[edit] Vizard's yellow book says for full race 1275 only, use red springs and RR needles.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.