This is some info that I recently got from a reputable company that sells both options when I was inquiring about the GV vs a 518.

"Having reviewed your tech submission and your request for information on our overdrive transmissions for your '70 Superbee with the 6.1 hemi, I offer the following information for your consideration.
As I had previously indicated, installation of the A500 or A518 will require extensive modification of your crossmember and floor pan modifications. In addition, the A500 or A518 will drastically increase the elapsed time on the strip by at least 1/2 second in the 1/4 mile, which in a street application will certainly be noticed in performance potential.

The option that I highly recommend for your application is our StreetPro 727A with the "Shortie Overdrive" option. This unit is only 7 1/4" longer than your 727 and is a direct bolt-in (other than driveshaft shortening). There are no floor modifications or crossmember fabrications required to install it the '70 B body. When installed, it will have fully automatic overdrive shifting functions like a new car. We have seen mileage increases as high as 50% with this unit. Dropping your cruising rpm drastically reduces engine wear and maintenance. This unit does not substantially increase power drag (slower e..t.) like the A518 Chrysler overdrive.
The "Shortie" version of our Street Pro series transmission has 4 direcct clutches and the 4 pinion planetary to take the extra load of the overdrive, which will also accomodate any future plans to increase your torque and horsepower up to the 500 h.p. level. For greater details see our website."

Just wanted to share and have no interest in this company.