ahhhh.. a very old school mod... i take it you plan on doing this to a 250x/300ex. very good choice, i cant beleive anyone else knows about this...
i used to have that mod done to my 91 250X with a TC built 350X mx-er. mine used a +1 extended swinger by leagers and a pep revalved 350X shock... very bad ass set-up. it also used 250R peps up front with 250R +2 a-arms... ahhh i miss that wheeler... but anyways here waht you have to do...
get a 350X swinger and shock, and get 5 washers that fit over the swinger bolt (i dont remember the size), i beleive there was 3 on the chain side 2 on the other, but im not positive, you will have to check the chain alignment. but i know for a fact it was 5 washers spaced 3 on one side 2 on the other.
the shock will bolt in factory location, i mounted the rezzy on the lower subframe rail on the chain side like a 250R was.
the stock 250/300 bearing carrier, caliper, caliper holder, brake line, and axle will work.
if you dont have a extended axle the 350 one would be one to look into being that it is either 1 or 2 inches wider than stock...
hopefully i helped you some..
if you got any more questions feel free to PM me!