Your t-stat is an obvious issue, either it is old and failing or someone drilled a bleed hole in it. Get a 205 stat for MPG 195 for a compromise and 180 if you want more of the same.

Also seems like you had possible trans issues when you bought it, that can kill the mpg also. Might want to do a full service on it, fluid, filter, adjust bands, tighten line pressure a bit, type F fluid. Can you feel the lock up engage?