It turns out that as long as you have a 4.3 slide and outer barrel (I wouldn't be surprised if NINE BALL is releasing these as aftermarket items as well), then it is a direct fit onto the 5.1 frame...
This is useful for anybody wishing to build a 4.3 Strike (meat basher front end) because NINE BALL has released a 4.3 Strike kit:
Key note is that this kit can ONLY be attached to the 5.1's mid-frame as it has the mounting holes required. The actual 4.3 has an integrated rail, and as such, can not fit the Strike kit.
This 4.3 strike kit will require the 4.3 slide and outer barrel.