I guess the small vendors on this forum don't do custom clam-shell adaptors? Someone here used to offer that service.
Yep, shift kits work well in my situation. I have a LOT of impatient car drivers around me in traffic, especially rush hour. If you're slow, especially from a standing start, they're quick to blow their horns.
To put the cassette's overall gear ratios into perspective,
first gear is like a Happy Time 89-tooth single sprocket;
second gear = 68t sprocket;
third gear = 60t sprocket;
fourth gear = 53t sprocket;
fifth gear = 45t sprocket;
sixth gear =40t sprocket;
seventh gear = 34t sprocket;
eighth gear = 29t sprocket.
So using a 40t HT sprocket is like pedal-assisting in sixth gear on a bike with a shift kit.
Shift kits are easier on the clutch too.