Bluejacket's smallest blocks are labeled 3/32" too - but every manufacturer seems to have a different way of measuring such things. The smallest of the Bluejacket cast metal blocks are the smallest manufactured blocks I've encountered. It's also relatively easy to make them even smaller - with a few strokes of a file.
The deadeyes of the Cutty Sark always present a problem to the modeler - especially the ones for the lower shrouds. In reality, each deadeye is surrounded by a forged iron strop, which has a small eye formed at each end. The ends of the strop almost, but not quite, meet at the bottom of the deadeye. The chainplate consists of an iron bar with an eye formed at its top and an iron plate, with two holes in it, at the bottom. The eye goes between the eyes of the deadeye strop, and the strop/chainplate assembly is secured by a bolt and nut. The plate at the bottom of the chainplate is riveted to the inside of the bulwark, near the deck. The chainplate meets the bulwark at an angle, so it effectively points directly at the lower masthead. (A verbal description of all this makes it sound considerably more complicated than it actually is.) A hole is cut in the pinrail, through which the chainplate passes. The result is that the deadeyes look like they're sitting on the pinrail. They really aren't; they're fastened to the bulwarks by the chainplates.
Reproducing all that in 1/150 scale is beyond the capacity of most of us. (It's certainly beyond me.) There are various ways to fake it - all of them involving getting rid of the plastic parts. One approach would start with drilling a series of holes in the pinrail at the appropriate locations. Pass a piece of thread or wire through one of the holes, around the deadeye, back down through the hole, along the bottom of the pinrail, up through the next hole, around the next deadeye, back down through the hole, etc. If you like, you could fake the chain plates with pieces of wire running from the pinrail to the base of the bulwark - but on that scale few people would notice them.
A big virtue of that approach would be that it would let you set up the deadeyes on the pinrails before mounting the pinrails to the bulwarks. The big drawback would be the amount of faith you'd have to put in the joint between the pinrail and the bulwark. If that sucker comes loose late in the building process....
A stronger solution would be drill the holes in the pinrail as before, but mount the pinrail on the bulwark before rigging the deadeyes. Also drill a hole through the bulwark at the point where the chainplate is supposed to be riveted to it. Tie a piece of strong thread around the deadeye and trim off one end. Pass the remaining thread down through the pinrail and out through the hole in the bulwark. Put a drop of superglue on the thread where it passes through the bulwark. When the glue's dry, slice it off flush on the outside. A careful paint job will make the fakery invisible.
I don't know how the Academy hull halves are made. If they're like the big Revell ones, the waterways (which actually are iron, covered with cement) are molded integrally with the hull halves. The best solution of all might be to warp reality a little and have the thread "chainplates" pass through holes in the waterways, to be superglued underneath. The drawback to that one is that, once the hull and deck are assembled, you'll never again have access to the bitter ends of the "chainplates." But if you do the glue job carefully everything should be fine.
What's absolutely essential is that whatever method you use to secure the lower shrouds is extremely sturdy. Those lines, even in a small-scale model, are subject to a good deal of stress. If they come loose, you have a mess on your hands.
The foregoing are unqualified musings of a semi-senile brain that hasn't seen the kit in question. As such, they probably ought to be ignored. But I think you can see the basic point I'm driving at. And I certainly do recommend ashcanning the plastic "shroud and ratline assemblies" that come with the kit. Several threads here in the Forum describe techniques for rigging ratlines; it really isn't as hard as newcomers often think. And the Cutty Sark, with only five lower shrouds on each side of each mast, doesn't require nearly the number of knots (or passes of the needle through the shrouds) as, say, an eighteenth-century warship.
Hope that helps a little. Good luck.