My 70 Duster build

Looking good!

Two Suggestions, I'd move the Oil filter up just a bit.
And you might consider a tubular x or diagonal brace for the core support. As you just cut away a bunch of the support in a area that is fairly flexible anyway.

I have an oil cooler that might work for you. It is off a Dodge ram. We replaced it at the shop I work at because the bracket was bent. It is long and skinny. I'll try and measure it so you can see if it will work.