in the past Ive used a 6mm wide seal sanded to 5mm wide no problems, there is plenty of room between the seals you just need to make sure the impeller is clear on the water side
FWIW i now use a 2007 KTM65 seal it seems to be made of a nicer silicone rubber
46035056100
Description: SHAFT SEAL RING 10X18X4
Retail: $9.39
Price: $7.51