If it was smoking before then my money is on the rings, if you had a couple of bent valves you would of thought that would have been noticed when lapping them in.

Blow by test meassures exactly what is getting past your rings by monitoring the crank case breathing using a blowby flow meter attached to the rocker cover.
You will of course need to know what a healthy engine should read to be able to compare the results, and I have no idea what they cost.

FRST Bottom end is a good move as you say with CPS ready for EFI at some point, just make sure you have the bores measured before splashing out on new rings (rule of thumb is that if you can feel any lip at the top of the bores then you need to go ovresize), also get the crank meassured and fit new big end and main shells whilst your at it! Yes the clutch will bolt straight on!