Transmission cooler question.

So I’m setting myself up to take the 904 out of my 70’s van and drop in a 46rh for the overdrive and lockup. It will behind a stout little 318. I don’t know the numbers yet but after build it should be 300hp. Not ment for racing just Cruze with a good sound from the side pipes.

And I have to upgrade the trans cooler lines to 3/8. I have a brand new Derale stacked transmission cooler (20,000btu) I was going to use ( been sitting on the shelf for 3 years now) but not sure if it is large enough. On paper it is; the gvw of the van is 6010lbs. Multiply times 3 (safety factor) and I’m at just over 18k. This cooler is rated for 20k but it is kinda small looking. Anyone else care to chime in who knows what they are talking about? Thanks in advance.
Syleng1