That's basically what I have, mounted up high on the firewall. Mine also doesn't hold pressure after the pump is off and I have to either key it twice or change the prime time to refill the lines. I think having the regulator mounted as low as possible is what you want, that way gravity from the fuel in the rails pushes down one side and gravity feed from the tank pushes down the other and the system doesn't drain out when there is no pump pressure.
For the transmissions, I believe that 6L80's, 8L90's and the 10L80/90 all use a 32-spline output and the same 6-bolt transfer case mounting flange. They all use different computers though and the 10L computers are usually encrypted from the factory unless you find an early 2017 version. Holley does not have a solution to control any of these transmissions. Zero Gravity makes a stand alone transmission controller for the 6L80/90 and the 8L90.
TCM-2650 Transmission Mechatronic Controller 6L80E 6L90E 8L90E. Used to get GM 6L and 8L transmission running behind most engines
www.zerogravityperformance.com
I would be VERY careful about mixing and matching an engine and 10L transmission. There are many combinations that will bolt together but very few correct: engine harness, engine computer, transmission, transmission wiring harness and transmission controller that will function without a massive headache. If I did a 10L transmission (and having driven them I really want one) then I would get the full engine and transmission, wiring harness and both computers from a Pre-2020 GM truck. T-93 controllers (2020+) are no bueno, T-87a can be unlocked, T-87 are not locked, but are 8L90 only.
HP Tuners is pleased to offer brand-new GM T87A TCMs for purchase. This TCM is found in various 2017+ GM vehicles with 10-speed and 8-speed transmissions and 2015-2019 GM vehicles with the 6-speed transmission. Good news—The GM T87A TCM no longer requires an upgrade! Users can now simply...
www.hptuners.com
If you keep the drivetrain intact you may not need to unlock your transmission or even engine controller at all. The more modules you use, especially the body control module and the gauge cluster and you can fool the computers into thinking they are still in the "host" vehicle. That's not really an option for me in my FJ45 because I have no need for the BCM or the fancy gauges, so I would have to at least unlock the engine controller to remove the vehicle anti-theft programming (VATS).