"Period correct" and legal for Bonneville are two ends of the spectrum. You can be true to the 1950 time period for engine technology but to be legal for Bonneville today there will be no doubt your tank is modern. I tried to keep my bellytank as true to the period as possible, but their is no doubts once you peek at the cage under the skin.
As far as the engines, the 8ba is easier to adapt transmissions and buy an off the shelf blow proof bell housing. But the 59A is slightly more compact around the flywheel on the bottom side, but you will need to build your own flywheel shield. In the end it may come down to what you have. I use the 59A series in my bellytank.
In any case first thing is to get a rulebook and study if you haven't already. Also, check out landracing.com forums for a lot of good info.
If you would like to discuss bellytanks and Bonneville send me a pm. Also, below is a link to my Flikr page with some build and finished pics of my bellytank for reference.
https://www.flickr.com/photos/vonwelker/albums
Andy