I think some of that depends on what all you want to do to the car in the long run or potentially have to take out to redo. Since you already have the auto and it's pretty much a straight bolt up I don't think it would be a bad idea to toss it on and have some fun with the car while you save up for the trans you really want. I swapped my Hemi in with a carb and MSD and had the whole thing on the ground having fun a week later. Drove it that way for a few months before I stumbled on a Megasquirt setup and swapped back to fuel injection.
I've seriously debated a T56 swap on several ocassions for the past few months, but until the price is what I'm willing to pay I'm not going to be doing it any time soon. I know I'm going to have to pull out the interior and cut up the floor, but I've been driving the car for over a year now and having a blast while I save up or wait for a deal. If you don't want to or can't pony up the cash right now for the trans you want and it's going to be some time before you can there's no reason to not enjoy the car in the meantime.
