In addition to what has been said, for the Fiero SBC adapters, the overall design of the adapter plate must include solutions for relocating both the starter and oil filter and the starter/oil filter bypass must be able to co-exist. The starter also must clear the cradle crossmember and not interfer with the exhaust manifolds/tubes. This side of the engine is very tight. In addition choices on overall ring gear selection and diameter will also come into play since the 153 tooth traditional SBC ring gear will not fit within the FWD transmission bellhousing (requires a 1" thick adapter to keep the ring gear out of the tranny). To go thinner, you must use a smaller diameter ring gear (or machine the tranny case for ring gear clearance).
Overall adapter plate thickness also impacts clearance between the harmonic balancer and the passenger side frame rail. Every 1/8" you add to the adapter plate, you lose at the frame rail. This framerail clearance also potentially drives relocating the transmission to the driver side - and once you pass about 1/2 to 3/4" you are looking at custom axles. So my personal preference is to keep the adapter plate as thin as possible while ensuring all clearances are kept in check.
Pages 1 and 2 of my 4.3 Swap thread have a lot of pics and discussions on clearance issues you need to contend with when using the Z-style kit (thinner adapter and Nissan ring gear) and the 4T60.//www.fiero.nl/forum/Forum2/HTML/087174.html
Now, if you plan to use the G6 6 speed with the SBC, things are slightly different. Since the transmission was designed for a thicker dual mass flywheel (about .8" thicker), the overall depth of the bellhousing was increased and the input shaft does not stick out as far. This means that for most applications you have to figure out how to fill this space either with spacers or thicker flywheels. Now if you designed an adapter plate just for the SBC/G6 application, the SBC crank already protrudes an additional 11/16" vs. the 60 Degree V6 family. The friction surface on the G6 flywheel sticks out about .8" further than a traditional single mass FWD flywheel, so in theory an approximate 1/8" thick adapter plate will work for the SBC/G6 combo with a flywheel designed for the application (has to be able to fit within the tranny and place the pressure plate fingers in the right location). Starter stability would need to be addressed, but shouldn't be a major issue. This would also gain 7/8" of additional clearance possibly making axle selection easier or providing enough room for a serpentine belt setup without notching the passenger side frame rail.