I have used, and use, my Daystar Quark on a variety of classic refractors, chiefly my Zeiss Telemator, but also my 80/1200mm Vixen achromat and 85/1575mm Zeiss A apochromat.
If the focal length is too long, you can use a focal reducer in front of the Quark, to reduce the focal length and image scale. This might need quite a bit of inward focuser travel, though, and not all instruments come to focus. An alternative is to use the Combo Quark and use a weaker barlow or powermate. The resulting focal ratio should be around f/30-32 or slower, so if the scope is f/15, use a 2x barlow/powermate. The original Quark has a built in 4.2x telecentric barlow, so a f/15 refractor ends up as a whopping f/63!
But the long focal ratios has a benefit: The bandwidth gets narrower and this increases contrast! You can also use a reducer AFTER the Quark, but the field then gets very narrow. Contrast is preserved, however, so this can be the way to go.
One thing to be aware of, is that the Quark is a bit heavy, so the focuser must be up to the job. A weak focuser will tilt the Quark relative to the optical axis and this will shift the observed wavelength, just like the tilt tuner on some front-mounted etalons. For this reason, I am particularly fond of my Zeiss Telemator as a solar H-alpha scope, as the accesories are solidly threaded onto the visual back and there are no moving parts, as the focusing is done by moving the objective.
And yes, the original Quark, because it has a telecentric barlow, allows you to come to focus with a binoviewer! And this REALLY brings out the details. The Sun is surprisingly low contrast and a binoviewer helps a lot here.