The od box is a great solution for fuel economy. But there are two minimum requirements; 1) enough engine to pull it,in your chassis, and 2) a matching rear gear to make it happen. The engine needs a lotta torque at a very low rpm to make this work.
The 225 is a much better candidate for success.And the low-mileage one,even better.A lot will depend on the rear gearing.And on your chassis;the heavier it is and the more wind it pushes, the less is the chance of success.
You have three things going for you;1) the bigger 225, and 2) the lightweight early-A, and 3) a small frontal area to cut through the wind. I think it will work. Perhaps a minor rear gear change to fine tune it,time will tell.
A 5-speed is a much better solution for these small engines.Actually, for almost any /6 or SBM combo
I once ran that box several years with a GVOD behind it,and enough SBM engine to pull a 2.02 final drive. I tuned her up a bit and got 32mpgUS. The same engine with 3.55s and a regular box could do just a bit better than half that,reving at about 65=3000rpm. With the 2.02combo, the revs were about 1600=65mph
