Whats the dimensions of your board? I still don't have an SVX ecu, but I've got a EJ20 ecu and it would be handy if the board would physically fit in both. You've probably done this, but if you haven't print a 1:1 image of your board, cut it out, and check that it physically fits where your expecting it to. Something so simple can save you some real money.
Ooo... one other suggestion, I just noticed this, don't lay the board out for even/odd arrangement. Save yourself the trouble of having to split the bin everytime you make a change and go ahead and waste half of each chip and set the board up for the same image on both chips (you've got the space to waste with 27SF512 chips). To do this just pull the A0 of what you've got designated as your even chip low, and the A0 of the odd chip high (you can tie them directly to gnd and +5V), and bump each of the address lines up one position (so A1 now goes to A1, etc). So each chip only gets half of its data read, but because your using a 27SF512 instead of the 256 its no big deal. Actually because the ECU is only using the top half of the memory space you really have the room to spare with the 256s anyway.
I would add bypass caps on the roms too, it doesn't cost you anything to do that.
Edit- whats going on with the A0? Why are you passing that through to the 27SF512s A0? Take a look at Kashima's schematic- althought he's latching it with one of the '573s he doesn't actually pass it to the '256s. He passes A1 to the roms A0, not A0 to A0. When the ECU reads from memory it always expects the even bytes to appear in the lower half of the data bus, and the odd bytes in the upper. So if you pass A0 your going to flipped flopper that arrangement unless when you burn the chips you double each byte (so on the even chip the same byte is read whether A0 is high or low, and the same for the odd chip). You could set your '512s to still work correctly with that, but thats complicated.