Ah, I take it that your Mercury is a MK3 or later with an articulated cocking link John ?
The MK1 and MK2 Mercuries had a narrower one piece cocking link but you'd need to use a MK1 or MK2 stock with it due to them having a longer cocking lever slot in the fore end.
Personally, I'd just widen the slot you have with a Dremmel.
All the best Mick