I'll preface by saying I will never be a buyer of an aftermarket block. This is strictly because I'll never build anything that requires additional strength beyond factory blocks. That info aside, I suggest 2 things. Aluminum block, and both Chrysler and Chevy bellhousing bolt pattern. Aluminum is self explanatory. Almost all high level racers that would use an aftermarket block use a GM based transmission. Why not simplify things for your future customer and give them the ability to use a standard GM housing without an adapter housing. This allows them to have easy access to buy something from someone if they have a failure at an event. I'm sure I'll get hate for this recommendation, but in reality, how many people running 8s or faster with a gen 3 hemi use a Chrysler trans?