You are checking your fluid level with it warm and running in park right. cold and not running will always give you a false reading on the level. With sitting for ten years it is possible the modulator or valve body is sticking a little.
If you do decide the trans is smoked and want to try a dirty trick to fix things pour a few ounces of dot 3 brake fluid in. It does a couple neat things, Works as a detergent to knock loose anything that may be hung up, it also has the neat effect of causing the fiber bands to swell a little bit to stop slipping. In our younger years we used this trick often to "refurb" the tranny in some pin half pulled grenade we where trying to sell. In fact I know one particular 78 pinto that got treated in said manner and went on to go another 30k or so before the motor gave up ahead of the doctored trans.