Does anyone know if there is an off the shelf hose that'll work or will I have to get one made and on the return side the nipple on the '66 box is much larger than the one on the back of the pump? Any ideas? Thank you for your time.

I adapted a 2000 5.9 magnum ps pump to my 65 coronet.
I had a high pressure line made. took both ends to the shop and they made me a new line.
wasn't much more than a new off the shelf high pressure line.

For the return line, you have a couple of options.
Both ends (at least for mine) are replacible. The ps nipple unscrews, and the top where the high pressure and return on the box can be removed.

IF you touch the one on the box, you will have to adjust it to prevent self steering.

Or you can get a brass adapter to take one hose size down to the other.