That's my "over the top" motor -- it's 200 HP and still fully streetable. More could be done, but that's as far as I've gotten it so far. There's a long list of modifications...

I don't know current prices on new Weber sidedrafts, but I bought a good used set for a different motor three years ago for $500 including manifolds and air cleaners. The manifolds required a lot of rework to fit and flow properly, which I did myself. Then you'd have to invent throttle linkage, convert to a mechanical fuel pump or low-pressure electric pump (no big deal), and very likely spend at least a few hundred dollars on venturis and jets to get them tuned decently.

D-jet is perfectly capable of supplying a stock motor with everything it needs, so that's why I say there's not a lot of gain to be had from the Webers in that context.

There's a range of cheaper "performance" stuff available that 1) makes less power than stock, or 2) provides mild performance gains at the expense of tractability. That's why I think there's a danger of spending more than a little, but less than is necessary to end up with a satisfying performance engine. I've done that, too...

1968 was the last year for the B18. There were carbureted B20s produced concurently with the injected ones. You can fit those carb setups on an injected motor, but it won't make quite the power (still better than the Weber downdraft, though). There's a pretty common big-bore kit that takes the B18 out to two liters, but you need a B20 head to make it work properly, or extensive reworking of a B18 head. Might as well just start with a B20 instead of overboring.

What is pretty easy to improve -- and improve greatly -- is the handling. Good suspension parts can be had from IPD and Vintage Performance Developments. That stuff makes the car a lot more fun to drive, and you won't miss having more power as much if you can carry more speed through the turns.