At the time I did mine__don't ask, had to be >35 yrs ago__it was referred to as a "competition TX core."
Essentially, you're looking to get the most fins per sq/in that they can fit within the fore/aft depth of the stock tanks. All the recored ones I've seen have had four (4) rows of tubes (I've seen OE MKIII radiators with 3-2-3 staggered rows of tubes).
The radiator shop guy will know what you want, when you explain that you NEED the maximum amount of heat-transfer/surface area available within the confines of your OE tanks and sides.
Here's a truly 'orrible stock MKIII 3-2-3 rad:
I replaced it with this: