Another DIY Led Question (Complete Newcomer)...sorry
You are, presumably, talking about an HLG 40-36  http://www.meanwell.com/search/hlg-40h/default.htm right?

Look at the data sheet. It can regulate current over a range of 21 to 36 volts. Since you don't know the exact model of your LEDs, the best bet is exactly as you suggest - use 8 in series. If you do that, you must use the dimming function on the MeanWell, and set the current to less than .75 amps. If you don't do this, you seriously risk destroying at least one LED. (Of course, if you do, you'll know you need to crank down the current, and you have 2 spares. Although I doubt that's much consolation.) As it happens, you can drop the current as low as 0.67 amps, so you're good there.

In principle, you could have a go at using two strings of 5 in parallel. In the worst case (3.5 volts per LED) you would only pull 17.5 volts, which is less than the minimum output voltage of your driver, so you would have to add something like a 10 ohm, 10 watt resistor in series with each 5-LED string to drop the excess voltage, but you would want to add such a resistor anyways to keep one of the LED strings from hogging all the current due to some of them needing a lower voltage than others, so that's two birds with one stone. Doing this would provide about 0.56 amps to each string (with the MeanWell set to a full 1.12 amps). Whether this would be a net win or loss as far as total light goes is probably a wash. Assuming equal efficiency at either current level, running 10 LEDs at the lower current would produce 90% of the light of 8 LEDs at max current, and this is essentially unnoticeable. Running the LEDs at lower current will make them more reliable, and will be much easier for you to provide adequate heat sinking for them.