The Van Dieman conversion would be a good one and pretty quick.
The "Build your own Sports Car " book is a load of crap for any number of reasons and should be used as a guide only.
1. We had a shed full of Datsun 1200/120Y/Sunny bits that cost us zip and we still parted out with about $2k before we had a driveable car. This is not a lot of dough to get that far really, but 250 pounds---forget it.
2. The design of the "Locost" has a number of fundamental flaws, not the least of which is the inbuilt tendency for bump-steer. This is curable, but would take some horsing about with the suspension.
3. The way that the car is built leaves it open to distortion and twisting during construction, more so if it is gas welded. I am convinced that the only way to fly is to make a simple jig and build everything off that. There may still be some minor distortion, but it can be covered.
4. I point all this out as a result of personal experience as this is the fourth car I have built and I am presently planning the fifth, which will be rear engined. If you need any further information please ask--there is no witchcraft to them, so there's nothing worth hiding.