400 degrees should be plenty hot enough although I have used 'cleaning cycle which ges up to about 550deg f

Did you remove the 'o' rings in the cylinder base before you started? (and the gasket)
I dont bother to pre heat, just stand cylinders on a baking tray and wait until the block drops off the liners. pair of welding gloves and pull liners out of block.
If you do both blocks at the same time is a good idea to
1. mark positions before you start
2. let liners cool down before you re-fit them to hot block
I have a 400F block I'm going to be working on in next few weeks, liners are out, not sure whats going back in yet

, got some 550 pistons but no liners

PJ