66 dart rear end swap

I have a 66 dodge dart and I have what I believe to be a 7 and 1/4 Rear end and have found that it will not hold the power of the swap I am doing from slant 6 225 to a 318. I was wondering what rear end I should use. I was thinking a ford 8.8 might work but I don't know how I would go about shortening it or anything. So what rear end would you recommend. It's My first engine swap and needing some help. Also if you know where to find tranny mounts for a 727 that would be great. But basically what's an easy and cheap way to put a new rear end in my dart. I am guessing the 73 f250 rear end I have won't work lol.
Ya, your F250 rear probably has 8 bolt wheels. Don't even go there for a swap. Do like Dana suggested or use search here for Ford 8.8 swaps. They are large bolt circle and will be a different bolt circle than your fronts unless they also have been changed.