I have been helping a mate here in Australia with an air head BMW for the same class.
It looks like you have taken a chain off the diff to the rear wheel. Unfortunately a lot of HP is used in the diff.
My mate has turned the engine 90* and used a TT Industries Norton box with a belt primary.
Turning the engine has not helped the width but it is a more efficient drive train.
Last year at Gairdner he ran a very easy 135.
We will see how it goes in a few weeks.
Like Wobbly said-- If you could move your foot rests back and also your seat, you may be able
to flatten your back, better aero,
I will get a picture and see how I go loading it.
cheers Bones