Q: Is it true that you have to cut into the firewall or extend your front end?
A: No, there is no need for cutting into the firewall or extending the front end. If you wish to use the stock engine fan that attaches to the fan clutch assembly, you will have to re-locate your radiator or extend the front end. This is why I recommend power fans for extra cooling.
Q: Can you use the Stock RB25DET mounts?
A: Yes, the stock engine mounts are bolted to the stock crossmember which shares the same bolt pattern as the KA crossmember when bolting up to the 240SX chassis. The tranny mount can be used, but must be taken off and flipped backwards. After you do this, you should add a spacer 1.5" thick made out of anything such as wood, metal, plastic, rubber, etc... as long as it will not collapse.
http://240skyline.nissanpower.com/custom.html
I'm going to assume the RB20 would be the same.