R75/5 with a Velorex. The Velorex was quite light, so much so that even with a car battery behind the sidecar seat there were places on the Washington DC beltway where I'd fly the sidecar at highway speeds. Scary the first time it happened; but it certainly worked to get the cars behind me to stay waaay back
. I did not have a subframe, so there was noticeable frame flex. The front trail was stock, so it took a fair amount of muscle to ride on twisty roads. The gearing was stock, but it was not a really an issue at anything but highway speeds. Going up long hills 4th gear was simply too tall, and I'd end up having to run about 50 MPH in 3rd gear. Not a big deal, but I always kept a real close eye on the cars coming up behind me to make sure they were paying attention.
If I was going to do it again, I'd start with an R100 or K100, or a /2 conversion (a /2 with an R90 engine). With the amount of work it takes to get a /5 properly set up for a hack, you might as well start with a bike with an engine that has the power needed to comfortably run at highway speeds.