yes the 500 is a pretty good tranny, you can get an input seal for a 700 and it will fit the 500, yes use some gear lube in it, do not abuse it and it will last, also the input bushing support that EC sell for around $18 will go a long way in making the tranny live, again it fits the 500 as it does the 700 series, although from what i have seen it fits pretty tight in that bushing, a friend of mine had to basically lap the bushing onto his 500, but works well!
and when and if you come across a 700, its a bolt in swap!!