OK. Now, with the chipset default values loaded as well, it should be basically working.
But the only BIOS I'm getting to boot on it is the Turbo XT BIOS, which has booting problems(floppy containing MS-DOS with flashing software from IBM for the BIOS I need, made on the i440fx). Also the 8042 isn't setup correctly, missing IRQ for keyboard as well as leaving it disabled.
Kind of a chicken-and-egg issue here.
Edit: Managed to get the i440fx chipset mapped into the i450gx chipset. The i440fx responds normally, with the i450gx only responding to the PAM registers. Both i440fx and i450gx PAM registers are shadowed onto each other when written. So putting any memory area in RAM mode does the same on the opposing chipset. The same for PCI mode. The only exception is that when writing the i450gx PAM registers and setting it up for unmapped mode(unmapping the RAM isn't supported on the i440fx chipset), it will map and shadow the i440fx PAM registers to PCI mode instead (which is the closest thing in this case to 'unmapped').
Edit: Just modified the RAM-mapping PAM registers to properly come from the MC chip instead of the second PB chip. So it was retrieving and writing the registers for the high memory area (4GB-8GB) instead of the proper MC chip ones.
That should fix any RAM mapping issues with the i450gx and i450gx emulating a i440fx.
Btw, the MC chips are the Memory Controller chips. And the PB chips are the PCI Bridge chips.