OP:

1) Never looked into it. Might have better luck with the phenolic core from the guy on here. its shorter than other vendor options.

2) Up to you, I dont like how any of the vendor FMHEs mount. a rad from a 92 golf with A/C would be a nice substitute. It'd let you keep the rad shrouding in place to promote better cooling.

3) kits come with gaskets, use RTV instead.

4) Depends how your transmission handles the power. Also depends on how much you abuse it.