chevy th350 sat for too long now wont shift?

Im my 1970 chevy truck, i just got it running again ( the timing chain broke several years ago, and everything went pretty well as far as repairing it until i had to pull the engine to get the oil pan back on, and i have been trying to get it running now for years. lol). But anyway, I tried driving it yesterday and the transmission doesnt shift. It just stays in low gear. It has plenty of fluid, and the vaccum lines are hooked up. It is possibly that the modulator got damaged, becasue it backfired hard enough to split one of the exhaust manifiolds when i was trying to start it a while ago, but other than that everything looks good. Of course, it ran fine yesterday and today it wont start again. Stupid truck. But anyway, is there anything else i should check as far as getting the transmission to work right again? Or should i be looking for a new one.

Is it original fluid ? Shift valves may be stuck in the valve body. The early transmissions sometimes had that problem. Some mystery oil fixed them. If you haven't already done so. A fluid and filter change would be a big help.

Easy fix. Governor is just stuck from condensation. put it up on ramps. on the driver-side just above the pan you should see a domed cover ~ the size of half a tennis ball. Unsnap the clip and pull off the cover you'll find a shaft with two fly weights on it spray them with lube of choice and wiggle them until they move free. pop the cover on and test.

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I had the engine out (to get the oil pan back on) like a year ago, so the fluid was put in sometime since then. I will check the governor next time i work on it, that would be nice if thats what it is.