Basic /6 costs for upgrades to a 70 Dart

Hi,
I"m selling a 70 slant six that I just posted today and I have never restored a car before but would like to hear what a ball park cost would be to upgrade a low options swinger to the 340 version or to even another 8 cyl that would make the car a bit more fun (nothing too crazy). I'm talking rebuilt equip/ engine/trans/ and anything else you know it would need to make that transition. Also availability of these parts and if you'd go to a rebuilt as first choice. My budget is probably about 2k. Would I just be better off buying one that already has an 8 in it and selling my current 6 cyl?

I"ve learned alot in a short time from this website but I'm pretty new at this. But the longer I've had the car in the driveway the more it is growing on me although I'm not sure I'd want to keep it in it's current form.
Any help is appreciated.
thanks,
Jay