iMac logic board upgrade

No can do, they will not physically work and as you have been advised iMacs are not upgradeable for the most parts. You can upgrade the RAM in many (not all) and doing anything else is really not for anyone but a well trained Apple Authorized Service Provider.

To be honest if you have a 2008 iMac you should send it in for recycling ( Apple Recycling Programs ) and seriously consider replacing it with a newer Mac.

The all-in-one iMac’s are not upgradable.

You can not upgrade the components on the logic board or fit a later logic board into them.

If you want or need a faster iMac with better spec’s, then your best bet is to just replace that one with a new or newer iMac.

It is not possible to upgrade an iMac to use a newer iMac's logic board. Every newer iMac logic board has a different port layout from the cutouts on the 2008 case and will not fit. Also such an upgrade is completely unsupported and any newer board may not work with the older iMac components.

If you need a newer logic board just buy a newer iMac.