If you already suspect you may have a slightly different PS3 model to the one pictured here STOP RIGHT NOW! Do not try and lift the motherboard off the lower casing if it is not coming easily. On mine, the cooling fan and heat sinks have extra screws directly into the lower plastic case and you will not be able to lift the motherboard out until after you have done Step 27. So skip this step, continue through the guide, skip 18-23 (you probably don't have a memory card reader). After unscrewing the 4 screws in Step 27, pull hard to separate the motherboard from the heat sinks (they will be attached with thermal paste like a glue). Only then will you be able to access the screws to separate the large fan and heat sinks from the plastic lower case.
Incidentally I totally smashed mine trying to get the lower case off here at step 15 - I broke the screws off, warped the metal on the heat sink and snapped pieces of plastic. But I continued and did the heat gun stuff and it still actually worked!!