I agree with Sparky
I know there's a long tradition of turning Lakesters into Streamliners - usually based on economics - but realistically, the reengineering that would likely need to occur in order to see this BEAUTIFUL streamliner happen deserves a clean sheet of paper.
If you're looking to keep costs down, I'm certain there are components that the two cars can share, but the advantages of a new frame would negate a number of the compromises you had to make for packaging into a bellytank.
And having done this before - creating a car out of whole cloth - you know now where the mistakes were made, and likely how to avoid the really costly ones.
Other than as a mental exercise, why compromise the design to an old set of parameters?
Dik, it's beautiful, and I'm sure you relished the challenge of creating a form that would work with the existing chassis. But think of what could be done if you weren't constrained by packaging issues irrelevant to a streamliner!
If it's the cost of the tubing that's the issue, consider the cost in frustration you'll encounter modifying the Lakester. You could well have to take it to the point of no return - and that would be a shame, because this is really one of the most elegant bellytanks on the planet.
I know this had been a plan in the back of everyone's mind for a while, but seeing what Dik came up with, I think the extra effort of creating a new chassis would be the best bet in the long run.