The T-19 trannies for the 460 does have different gear ratio's than our diesels. That being said though, the only other thing that makes our diesel trannies "heavy duty" is the number of balls in the input/output bearing races. On a gas tranny, there is 9, on a diesel there is 11.
I'm not clear on what you mean when you say you "lost" 3rd gear, but most likely what is gone inside is the synchro sleeve which allows you to shift in/out and holds the position when it's placed. There is little tiny "keys" on these things which wear out, blocking the ability to shift the sleeve.
The T-19 is easy to rebuild yourself,(I've done my own, and had never seen the inside of a tranny before) and there are kits out there, but since you have a gasser one, I'd probably suggest just find y'self a diesel one at a wrecker, throw in new in/out bearings and go, unless you specifically want the ratios you have.
I bought a parts kit off ebay from a company called "transmission exchange co." who sent me used bearings (you could see marks on them where they had been pressed on/off shafts and spun rough) as new. Beware, if it seems too good to be true, it probably is.
I wouldn't bother trying to change the ratios of your present tranny. It'd be way too cost prohibitive.
I have a second diesel T-19 (rebuilt) sitting in my shop right now, but I figure weight would be so much it wouldn't be worth shipping it anywhere.
You should be able to find a decent one at a wrecker for a couple hundred bucks...