This amp is quite flexible in regards to power supply voltage and load, and the heatsink requirements will vary accordingly. I'm using a +/-25 VDC supply and am driving a 4-ohm load. It sounds like my heatsinks are about the same as ih's, and I mounted them on the outside as well. They barely get warm to the touch, but I don't run at significant power very often.
Later, I'll change to a +/-42VDC supply to run an 8-ohm speaker. My feeling was that this higher voltage was a bit too much for driving 4 ohms.
I'm very happy with this amp. I'd go with the biggest heatsinks that are reasonable for your budget and the case you select; reliability will increase if you keep the temperature down.
I beleive Rod stated in the article that a 0.6 °C/W would work good for an stereo pair. I once found that kind of sinks for $20-25; if you get those prices, buying one for each would be a bit overkill, but if you have the $50 the amps would run really cool.
A friend of mine used a 1°C/W on his and the amp runned just warm.