I was looking to maybe have the 225 rebuilt, but there's no modern after market for it in the states, I have to order parts from Australia,
si I decided to up grade to the V 8 and a trans with auto over.
I'm going to take flack for this, but you made the right choice. Any traditional inline-six from GM, Ford or Chrysler are all weezy little turds.
Yeah, go Magnum V8 with a 727 automatic transmission.
Magnums were produced from 1992 to 2003. The V8's were 5.2L (318ci) and 5.9L (360ci). You can go either way and still be pretty happy for what you're trying to accomplish. If you want to stick the factory fuel injection then earlier is better as you'll be able to move the entire engine computer and harness to your car. The later stuff got nuts with the electrical complexity and would not be an easy swap. You could probably use a newer motor with earlier electronics, I don't know for sure. There is always the possibility of aftermarket EFI too. Holley makes great stuff and I'm using it on my car.
Your current transmission will not bolt up to a V8. What you're going to want to try and find is a 727 out of a 2WD truck or van. 80's to 90's. Go back to early and it won't have lockup; which is something you want in a driver car. No custom mounts are needed as will bolt right in place of what you have now.
At the minimum you'll need:
1: A new center sump oil pan and pickup. All trucks use rear sump pans.
2: Conversion motor mounts
3: A B&M flexplate balanced for Magnum motors to 727
The above will get the motor and transmission in the car. You will have to shorten your driveshaft.
Probably the biggest PITA is adapting a high pressure fuel system for the injection.
It's not a complicated swap, but do your homework and have someone that can lend a hand that is familiar with Chryslers from this era.