Torque plates pull the cylinder walls out of shape just like a head does when torqued down. Its not much, but enough to pull the cylinder wall out of round a tiny bit. For the normal everyday engine, you will see no difference in performance or wear. It really depends on the block design and how much material is around the bores on weather or not a torque plate will do much. We hardly see any difference on the V8 Pontiac's dad I build due to the blocks thick deck. The inline engines are fairly heavy blocks with small bores so who knows. Core shift (wall thickness) can make a big difference, but that requires a sonic test of each bore to determine where the core shifted to.
If you find a plate or make one, use it during boring and honing, my local shop will do it both ways, hone only or both bore and hone, they recommend both. Sometimes honing doesn't get all the distortion removed. They also charge extra to use a plate due to man hours of torquing and the extra handling.
With a large hole saw, drill press, plenty of cutting oil, and a surface grinder, a fellow can make one pretty easy. If you can find a flat cold rolled 1 1/2" or thicker piece of steel plate, you may not even need a surface grinder. The bore holes just need to be bigger then the final bore size so the tooling will pass though with out hitting. Be sure to use the same head gasket type for assembly as you do for boring, and be sure to lube and torque the head bolts the same. Any deviation from boring to assembly can change the cylinder walls making the torque plate useless.
I would assume you want to skim cut the block deck before boring so the torque plate will set flat. On the other hand, you don't cut the deck surface of the block till you test assemble the engine so deck height can be measured using the final pistons and rods. A good shop can adjust the deck side to side as well as front to back to insure its flat and square to the bores. A true deck assures each bore has the same compression and quench area.
Joe
