I'm assuming your cowl is the same cowpat style as my '55 GF. This is how I do it. Put the grommet in place. The larger side goes downwards. I grease the grommet ring using bicycle grease*, (OK with rubber & plastics). Push the speedo into the grommet up to the bezel. I made a tool by bending a small screwdriver blade to make a small 'L'. Keep the speedo lightly pressed down then use the tool to work around lifting the grommet rubber over the bezel ridge. Easy!
PS. I damaged the first grommet doing this but if you are careful it's OK.
* My speedo tends to rotate slightly over time so you might try without the grease.
