My Z has a tank where the airbox normally is. It sets under the airbox cover. I have no idea what a 300ex gas tank looks like but a Tri-Zinger tank will work. of course you have to cut a hole through the airbox cover for the fill inlet and run a clamp on type airfilter as the stock filter and airbox setup becomes ebay material along with the stock gas tank.
1986 Tecate
1984 Tecate
1985 Tri-Z
1986 ATC350X
1985 ATC250R (2)
1985 ATC125M (2)
1982 ATC70 - Original Owner
175 TriMoto Frame with Pro-Tec Rear Suspension
3-Wheelin' since '82