The hoses that come with the 678 are sufficient. You really don't need any hard lines.
Hard lines aren't needed, but they are often really nice to have for packaging and routing.
Indulge me, here, because my livelihood is essentially on-engine fuel plumbing and related components (pumps, injectors, filter, etc).
Because of the temperatures and pressures in a trans cooler line, a flex hose can easily satisfy the needs. Where a hardline really shines is 1) bend radius, 2) mounting/security, and 3) compact packaging. Hardline for the same flow capability is much smaller outer diameters, so it takes up a lot less space. As a general practice, some lines need to be able to move because the ends move-- brake lines, for example, have to flex with suspension movement. Fuel lines need to allow for engine movement within the mounts. But the converse is also true-- if it doesn't need to move then you should NOT let it. Lock that sucker down and it will be reliable.
I was actually thinking about this topic last night. Namely: how can I plumb a transmission cooler and use both hardline and soft hose to get the best of both worlds? What I often call a "hybrid" line has hose sections assembled with portions of rigid hardline to give you the best of both worlds. I have figured out a way that I think will work that allows both hardline and softline while honoring my personal design rule to never use a tapered pipe thread if at all possible.
Here's the key to making the hard/soft hybrid work:
The fitting at one end pushes into a regular push-lok hydraulic hose like Parker 801 or similar. At the other end, you have a swage-lok style compression fitting the works with rigid tube. Now, these aren't your normal compression fittings that will leak and make you hate compression fittings forever. No, these actually work and do not leak. This particular fitting is a Yor-lok version (from McMaster) that is basically a Swage-lok copy since the S/L patent expired.
With a fitting like this, you can easily hand bend some stainless tubing in 3/8" diameter or 10mm (they offer fittings for both metric and SAE tubing that both work with -6 or -8 hose).
Parker 801 (push-lok Plus) hose is good to 125C and 300psi in the -6 size. Now, keep in mind that temperature rating is a forever-and-then-some rating, and exceeding it by a small amount is not a large risk. Parker 7212 "jiffy" hose is nearly identical in but it has a lower grade rubber cover and is only rated to 100C, which is IMO okay but borderline for trans cooler duty. I'd steer you towards the 801 as a minimum.
The "good stuff" that I would recommend for trans cooler plumbing is Parker 836. It's rated to 150C (that's an industrial, all-day-and-forever rating, so it can handle a lot higher for somewhat infrequent duration).
So what would a Gucci trans-cooler plumbing install look like?
Well, I'd probably start with a Setrab cooler with the M22 ORB ports. Use the Setrab hose ends to adapt to Parker 836 push lok hose (keeping in mind that Parker's own 82 series fitting range is huge and can support all kinds of adaptation to push-lok). Route along front of core support, transition to rigid stainless 3/8" tube to snake tight along the edge of the radiator and tuck tightly in there with some nice secure screw-down or other kind of tube clamps locking it down. Then finish the rigid tube run with a u-bend to connect to the OEM radiator inlet with a short section of 836 to give a bit of flex and work with the OEM radiator barb. Transition to OEM hardlines with the yor-lok compression fittings.
I'm still sketching this out in my mind. I'd love to see if I can pull it off.
. They are the best mod you can do for a GX that sees heavy use of any type. When you are going up a long grade, just put the trans in 2nd or 3rd gear, lock it, and motor up without any additional heat built up.
