Hi Phil,
Yes, I did the same thing a while ago. There are a couple of threads here with some excellent photos from Joji which helped me a lot in figuring out how to put it all together.
http://www.lotuseuropa.org/LotusForum/index.php?topic=874.msg6460#msg6460and how the job went, or rather didn't go as I discovered the seat belt mounts in the sills were almost non-existent.
http://www.lotuseuropa.org/LotusForum/index.php?topic=883.0Towards the end of the second topic it gets onto the inertia reel belts mounted in the car. If you're wondering if the change is worth the effort, then yes it certainly is. The belt pulls clear of the door, there's none of that "lean in, move belt, get in, close door, open door to remove belt trapped between door and sill, curse static belts....."
The only thing to look out for is how the inertia reel is intended to be mounted. If you buy NOS then this might be a problem but a lot these days are able to operate in flat/vertical planes and that's what I went for.
Brian