I can't say how the trans will fit in the tunnel, but sure you would have to make some sort of new crossmemeber (not that big of a deal..)
On the diff, I'd stick with the 8.75 for sure. As for a speedo, you could go a couple of ways.. You could use an aftermarket electronic driven speedo off of the speed sensor on the transmission, or you can buy a little box that converts the electronic signal to a mechanical cable output and keep all the stock dash that way (A couple of companies make them IIRC..I've got one here actually for a swap I am doing)

There are a few Moparts memebers who've put new generation Hemi's into older cars, so that side of it should be pretty well covered for you at least.
Don't forget about upgrading your fuel system as well


(I am doing a late model engine into early car swap myself at the moment too...not Mopar though )