First let me say this, the 68RFE stock unit has fatal flaws in it from Mother MOPAR and you should not replace a trans with another stock trans. it will only do the same things. Buy an aftermarket rebuilt trans that's built to either a Stage I or Stage II level. Make sure the rebuilt unit has a billet input shaft, an aftermarket valve body and pump, and clutch packs that have more clutch and sheet discs in it. Also consider a triple disc converter.
You can find, online, a number of well known trans builders that know what they're doing and can provide you with either a Stage I or Stage II trans for between $4500 to $6400. Then you can install this or have it installed and have a long life with it.