Get the biggest 8V rad you can get. I believe this is the 25" wide core kind.
16v's are slightly different.
The good news is they are super easy to replace in a 8v.
Might as well put a 180 thermostat in there while you have it drained.
you'll swap the fan shroud, swap the thermoswitch, check the shroud flappers (important!)
the rest is easy peasy!
Might I suggest you open up the heater valve and flush the whole shebang before you put the new rad it. You'll be amazed at the guck that comes out.