For better clarity, LS.....when you say, "rebuilt", what did that entail exactly? Professionally completed rebuild with new Italian shafts, gears, forks, matched and de-burred zero-rock sliders, syncros and hubs along with a 1' counter shaft w/ external bolt-lock upgrade, 20lb shifter cam spring and of course bearings, gaskets n' seals - or (one extreme to the other) just bearings, gaskets and seals done at home?
"What's the difference if it's working good now as is?" A full rebuild in quality parts alone for an M20 is $1,500 CDN in my market area. Then add in labour. A box with ~48 year old gears and shafts......well, if it does work good now how long is that going to last?
Either way, a good, low mile "in working order" M20 should come in handy to $1,500 to as much or even more than $2,200 CDN range in various N.A. market regions. (Exchange is approx. $1.30 on the CDN/USD today so ~$1,000 - ~$1,700 USD).
"So, yeah all that but what's it worth?", you ask? To an individual that wants that complete, ready to install 4-speed......, as much as you want to sell it for below ~$2,500USD. Anything above that would push most folks off to a brand new TREMEC 5 speed for $3,200USD & change, conversion be dammed!
My local market (Eastern Canada) would be in the higher range of $2,200 CDN ($1,700 USD) if it is a sound working box with a full rebuild including brand new Italian gears and the available machined upgrades and a buyer who specifically wants a 4-speed Muncie M20. Only a very few would pay that around here.
I paid $700 for mine last year. It's a 27 spline M20 wide ratio and was rebuilt by my neighbor 15 years ago, stuck under his bench, then sold to me. I think mine was a great deal, so $700 would be a good minimum price for yours, I think. Maybe $1000 would be a good asking price?
Car came from Florida, so maybe Paul C. rebuilt it, and even used a marker to sign the guts. I have no ide what's in it. Other than a show car nut and bolt resto. I am confident it got more than 2 gaskets and a seal.