Thanks
vp1337 and
J.Higgins
For anyone who's interested in the science of how accurate indexing works between 7 speed Suntour Barcons and Shimano Cassettes...
Shimano HG 7 speed cogs are 1.85mm thick with 3.15mm spacers to equal 5.0mm center to center spacing. 8 speed spacers are 3.0mm.
Suntour Accushift 7 speed cogs are 2.0mm thick with 2.8mm spacers for the last three (low gear) for 4.8mm center to center spacing.
Sooooo Shimano 7 speed cog (1.85) plus 8 speed spacer (3.0) equals 4.85 center to center spacing and close enough for everything to work without fuss.
And to reiterate, you only need to use 8 speed spacers on the last 3 (low gears) of the Shimano cassette, since Suntour indexing has split spacing of 3 x 3.0 mm(H) and 3 x 2.8 mm(L).