A couple questions:
What's the front and rear wheel rates?
What's the front and rear suspension travel?
Reason I ask is because I tried using bike shocks, but starting with 1.5" travel and 600lb spring rate and couldn't achieve a usable wheel rate and retain good suspension travel. Since I wanted 6" wheel travel, that's an installation ratio of 4:1. That means I'd need a crazy-high spring rate - 16 times the wheel rate. This is why I'm interested to know what you did.
Do you expect oil leakage from the diff as the air inside heats up, expanding and pushing the oil past the seal?
Front Wheel rate is (from memory) 75 lb/in with the rear being 80 lb/in. I approached the wheel rate from a different direction based on my experience with my '70 Europa. That car had 100 lb/in front and 72 lb/in rear spring
rates. in stock form. I increased the stiffness approx 40% for autocrossing but I didn't like what it did to the ride quality. My Locost will be (realistically) driven much more on the road than on the track so I erred on the side of ride quality. I also went with urethane bushings on one end of each wishbone with Heim joints on the other end for the same reason. The rear suspension geometry of the Europa and my Locost are very similar so I felt that going with similar spring rates there was a good starting point. The Europa is also a relative porker at 1450 lb vs 900 for my Locost so I still expect the locost to be tighter than the Europa. The R1 springs have a rate of 510 lb/in and 2 inches of travel before they hit the bump stops. My rockers have a ratio of approx 2.5/1 resulting in almost 5 inches of travel once all is said and done. I figure that stiffer springs and roll bars are available for tuning if required as are different rocker ratios.
You have keyed in on an area of concern that I too have with the diff. I designed the can that encloses the diff with o-ring seals so I don't think it will leak there. The problem (if there is one) will be at the axles. I designed that with lip seals that are supposed to have some low pressure sealing capability but I really don't have any experience with them so we'll see. At least one DSR guy claims that the diff can be pumped full of grease to help it all stay put. The DSR guys have figured this out (or clamed to at least) so It shouldn't be impossible to sort out. If not I'll just say it's basically a British car and that you should expect it to leak.