if the problem is getting the tyre to seal then I recommend you fit it with a tube, pump it up to 30psi, leave it over night.
In the morning pop off one side of the tyre, remove the tube, insert valve, re-fit the other side then pump like buggery.
Worked on one set of Flows with Hans Dampf, bizarrely my other set of the same combo went on ‘dim problem o gwbl’.
If the actual problem is getting the tyre onto the rim in the first place then, as above, make sure one side is on and positioned bang in the middle of rim (the lowest point) then the other side shouldn’t be too bad.
Have to say that getting Hans Dampf’s onto the Flow rim was a piece of cake (ignoring the sealing process).
Maybe Crest’s are simply too small for that tyre??