OP, your slipping was probably the torque converter, not the trans. But is probably best to get it done at 250K if you plan on keeping it. Just make sure you go with a lower stall converter. It will a difference when pulling over the crappy stock unit.
I just spent $6k on my custom built 68RFE rated for 750 hp and $1.6k a fixed geometry S364 turbo with a T4 pulse manifold so I kind of feel your pain. Although mine was because I wanted to not, because I had to. Ironically I have friends who think I am nuts yet just paid $20k for new Harley's which I think is nuts. I would rather drive a 600hp truck then a Harley, but I guess we all have things we like to spend our money on.