This is do-able, but as Sailboy indicates it can be really tough to get those winches off if they've been there awhile. An impact driver may be required to remove the base bolts due to corrosion, assuming that the bases are sound.
The other consideration is to ensure that your proposed location on the cabintop is adequately reinforced to handle the torsional loads that grinding a winch can place on that panel. A substantial backing plate and maybe even a pad under the winch itself (which will also help the lead angle to the drum) will be helpful here. If cored make sure you don't compress the core and make sure the core is well laminated in that area.
Follow the usual good practice when drilling the bolt holes in the core: drill oversize, fill with epoxy and re-drill proper size after curing.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"