[Walt Fricke]

The twin plug machining isn't cheap - it is hard to bore through the head's fins without breaking some of them. A dual distributor is also expensive, so perhaps a better alternative would be some sort of crank fire setup, selling your distributor and blocking the hole in the case. Plus you need the extra (longer) wires, and a way to feed them through the engine tin to the lower plugs. Now spark plug changes are twice as time consuming, especially since you are now down under the car (and even with a lift a stock exhaust has to be dealt with). I rate the likelihood of power gain at zero, and economy at very little. Twin plugging is for highly modified engines (at least for those with significantly higher compression ratios. It might be worth it if you had a later Euro SC, which can be borderline on US gasoline at a nominal 9.8:1 CR.

If you have come up with a nice budget, and are determined to spend it, you might look into some kind of single plug crankfire system and sell your distributor to offset some of the cost. Those are said to smooth out the idle some. Or switch to one of the various electronic fuel and ignition systems, which you can tune for maximum economy if that's a benefit. If you have an inner geek, you can spend profitible time tuning the programming.

Sounds like you are happy with the car's street performance, which realistically is all any sane person really needs for street/highway/even curvy mountain roads.