I know that my value is basically as a card tester but I would like to try to clarify some things.

The MCUC table is needed to initialize the GDDR5 RAM used on 4870/5770/5870 cards. In the PC BIOS that starts the ROM is an address to "look up" the GDDR5 table. To use a "BASIC" analogy, this is a "GOTO" statement with an actual address to load those GDDR5 inits. The table itself starts with "MCUC" so that is your 2 instances, the first in the PC BIOS saying "Hey, to init the GDDR5 RAM, check out the specs at address 'WXYZ' ". The second instance is at address "WXYZ".
Between the PC BIOS and the MCUC table is a bunch of empty space filled with "FFFF"s. It is into this space that we place the Netkasized EFI. In the case of the 5870 I believe this EFI is 43008 long. You can not just wedge it in between where the "FFFF"s start and the beginning of the MCUC however. The PC BIOS has a length specified in blocks of 512 bytes. To find this number look at the very beginning of ROM. It will have a opening that starts with "55AA" the very next 2 characters are the length of the PC ROM. For argument's sake lets say this is "7A".
Download a nifty program called "Magic Number Machine" and hit the "Radix" button. Now you can easily convert from various bases to decimal. Choose "Hexadecimal" and enter "7A" now hit "Decimal" and you will see "122". Next hit the "X" to multiply 122 X 512 and you will get "62464". This tells you that the PC BIOS actually ends RIGHT before this number if you look at addresses in decimal. This also means that the EFI ROM needs to START at 62464. So if you want to see if it fits in there, just select everything from 62464 until beginning of MCUC. If you are in "decimal" view, you will see what is selected. If it is Equal to or more than 43008 (or whatever length the EFI you are working with is) you are good to insert it. If in fact it is LESS than 43008, you have more work to do. While using Hex addresses, note the address of the beginning of the 2nd MCUC.
I would suggest methodically selecting 512 bytes of "FFFF" from BELOW the MCUC at end of ROM and cutting it from there and inserting AHEAD of MCUC. If you are 512 bytes or less short of fitting to begin with, then 512 will do. If you are more than 512 short, than it will obviously take more. It is possible to take less than than 512 at a time, but I like to use that when possible since ROMs work in chunks of that size. When there is enough space, insert the EFI then make sure that the length has always stayed at 131072. If this ever varies, you have screwed up. It absolutely MUST start at 62464 (or whatever is specified after "55AA" in your ROM)
Once you have made space and inserted the EFI you will need to fix the MCUC lookup address. To do this you will need to use the HEX addresses by unchecking "USE Decimal Addresses" under Options. You find the spot before first MCUC where the Hex address is listed and change it to reflect the new location. Now run the "fixrom.py" script. It is CRUCIAL that it lists 2 at "OpROMs" and notes some "RAW DATA" afterwards. If it sees just one OpROM, I can guarantee that your new ROM won't work. It must also see the "RAW DATA" of the MCUC table. If all 3 of these elements show up in terminal output, you are in a good spot.
The scripts that Jeanlain has in opening post automate the instructions I have listed here, so my explanation is really for people whose ROM using his instructions isn't coming out right. It is very difficult to try to verbalize these things so if you have tried this several times and THOUGHT about it and still this isn't making sense speak up. I will help out as long as I don't feel the questions are from "I don't want to think, please spoon feed me" types.