I'm no expert on assembling 2 3/4 engines, but in general zero is not enough, and 0.020" is probably too much, so maybe aim for 10 thou or so? There is no adjustment per se, so you'll have to see how you go with/without a paper gasket between the crankcase halves, and with/without shims behind the main bearing outers as required. The key dimension is the centrality of the con rods in the cylinders. If you've had a con rod mischief, don't forget to double check that the rods are straight before everything goes back together.
Leon