I think they are extremely overpriced, go buy a Mishimoto core from eBay and take it to a tig welder, I paid a couple hundred for the core and a local welder 2 hours of labor, total cost was about 75% of the cost.
I precut the aluminum on a band saw and pressure tested it which checked out fine...my welder gave me assurances if it had any leaks he would touch it up for free, fortunately it held 50 psi for over a hour which was good enough for me.
Go watch 6061 on YouTube for how to work with aluminum, good stuff there.