The Velocette gearbox cover can be stubborn to remove with the 'box installed in the frame as it can be difficult to get any purchase on it to free the joint between the cover and the casing, particularly if the joint is stuck fast with gasket cement. The 2 hollow dowels between the cover and case may also be stuck fast due to corrosion etc.
To help break the joint in future I drilled the top 2 bolt holes in the cover (ONLY) oversize to 6.8mm, and tapped them 5/16 BSF, having first removed temporarily the hollow dowel from the rear hole.
The gearbox and cover are then re-assembled using the original 1/4 BSF bolts and 2 hollow dowels.
Then, when removing the cover in future, once the 7 x 1/4 cover bolts are removed, a 5mm dia solid dowel is inserted in each of the top 2 bolt holes. The length of the dowels is approx 1/4" longer than the depth of the threaded 1/4 BSF hole in the case. 5/16 BSF bolts are then inserted into the tapped holes in the cover and screwed in until they bear on the solid dowels. By screwing the bolts in further, the top of the cover is forced away from the casing, helping to break the joint. Care is needed to ensure that no damage is caused to the cover by applying too much pressure.
It may be found that the solid dowel in the rear hole needs to be slightly longer in order to stand proud of the hollow dowel rather than the joint face, or alternatively the 5/16 BSF bolt in the rear hole could simply bear against the hollow dowel itself provided the cover has been drilled and tapped true.
To date I have tested the system on a clean joint and it worked as intended but I have not tried it on an old established joint so can't say if it helps or not.