Upsizing a battery will not cause any problems, an usually will last somewhat longer, all things being equal. Did this to a V65 in 1986 and am still using it today with no problems. Used a GL1000,1200,1500 battery. Just casual glance says space is the issue for a Valkyrie. When my Valkyrie battery dies I will be looking at using the Goldwing battery if possible. Thanks Pete.
Stock battery box is long gone. I welded one up as part of the rear fender. Made lots of extra room. I needed more room for the EFI equipment.