The head sits nearly 40mm higher with the rb30 block under it. You can slot your engine mounts a bit, but the exhaust wull still be a fair bit closer to the floor pan. Usually there isnt much clearance so it will foul if you don't do anything about it. Have a look at how close your exgaust is at the front and see if it could go straight up 30 to 40mm. I made a 45mm thick flange to space it down, which also moved exhaust back a bit due to angle of the flange but that didn't matter.