an rb 25 head does bolt to a l28 but there are other complications for instace, you would need to change oil and water lines in the rb25 headto suit l28 block, change the head from belt driven to sprocket, then get new computers and get it custom tuned to suit the engine, new exaust ect how do i know this its because i was thinking the same thing it would give a great power increase and mods would be almost never ending
thats my 2 cents for the day
cheers pauly