Cliff, might I be so bold as to suggest a way in which you can do these projects so that results will give some excellent feedback for comparison purposes.
1. By all means use the Alloy Block, preferably with 4.125" bore.
2. Use a 289 crankshaft- 2.870" stroke- std cast or steel, wont really matter.
3. Chev SB rods 5.700" long- preferably aftermarket & a light version if possible.
4. Pistons with 1.070" pin height ( These are originally designed/intended for the 3.400" stroker versions of the SBF and are available in the 4.125" bore size ).
Now when you build your flat plane crank ( with the same 2.870" stroke ) & camshaft ( with the same lobe spec's as the original build ) to suit that is all you will require and be able to fit the new crank & cam to your existing block. Have specimen weights for the rods/pistons made up at the time of building/balancing the first engine. I have a feeling that crank & cam will cost more than the whole initial engine build, thats why I suggest the shorter stroke, less machine work/metal removal involved.
With the 4.125" bore & 2.870" stroke you will have 306 cu in and both motors will love to rev!