oh, i didnt see that part. dont drill your tank if you want mounting holes LOL!! what you do is get thin bolts (like 4x32 or 6x32 thread size for standard thread, maybe 3-5mm for metric), and you just screw them thru the radiators fins. make sure the bolt its longer than the rads depth (1 1/2" is good, but see how long you need em).
you'll need to cut out a hole similar in shape to this:
Code:
__________
-->* / \ *<-- holes where
/ \ the bolts go
| | through.
| |
| |
\ /
-->* \__________/ *<--
you then slide the rad (and the ends of the bolts) thru those holes, and tighten it down with a nut.