You could have a number of issues. Before GD says it, burn it and get something old school GM
Serious now... you could have an issue with your HGs depending on when they were done last; there could be an air leak in the system or a stuck thermostat - or both!
25 deg BTDC is a bit much isn’t it? The MPFIs over here were all set at 20deg with some plugs connected under the dash while adjusting so the computer didn’t mess with the timing while you were adjusting it.
Other things to look at, what’s the condition of your water pump?
It could be worth getting the nose as high in the air as safely as you can, run for a minute then let it cool down. Check the coolant level. If it’s down it could be worth pulling the thermostat housing to see what that’s doing. Test it in some water (hanging from wire in the water, not sitting on the bottom of the pot while it’s being heated). Replace if dodgy.
While that is out full the engine with coolant through the thermostat hole if coolant is needed. Squeeze the heater hoses to try and displace any air in there and top up as needed again.
Refit thermostat and rad hose, back fill rad hose and connect to radiator. Turn engine on and slowly/steadily fill the radiator with coolant. Monitor temp etc and continue filling with coolant in small amounts when it lets you. Throw rad cap on, hold some revs for 10-20 seconds, let idle then shutdown. Let it cool. Check coolant level and top up as needed.
The other thing to look at is your coolant temp sensor and TPS - make sure they’re working within spec and that the throttle cable isn’t holding the throttle open a little. If it is back this off and see if idle changes.
Let us know how you go.
Cheers
Bennie