To fit Roadholders to a Vincent UFM a machined adapter had to be used.
If this was to compensate for the Roadholder's headstem being too long you'd have a problem with the frame having a longer headstock tube than the Girdraulic's stem. Could the Featherbed head tube be shortened?
If it was because the Roadholder's headstem was a smaller diameter you may have difficulties with bearings.
In the absence of spare Girdraulics, I fitted the Roadholders to someone else's Comet giving no additional thought to how it affected the trail. No steering problems were ever reported to me, and I saw other Vincents with Roadholders fitted, so that may not be a problem.