The ST300 is a pretty tough tranny. I would top up the fluids, get rid of the old gas, put in some fresh stuff and go for a drive. See how the tranny shifts and if all is well then pull it and replace the front and rear seals, pan gasket and filter, put it back in and drive it. It will be a cheap fix you can do yourself to get you on the road while you save some cash to upgrade to either a turbo 350 or 200-r4. Hate to see you through a pile of money at the trans to find out the motor is toast.
