a friend of mine has an 84 Olds cutlass and has a 425 engine he wants to put in it. will the 425 bolt up to the stock (Auto) tranny?... and what other mods will need to be done to the car?  

I'm going through something similar with an 85 Delta 88.

The stock tranny will bolt up, but most likely will grenade when you try to get on it.  Big block Oldsmobiles aren't known as stump pullers for no reason.

Other than that, from what I've been told, the only issues really are making sure you have enough hood clearance by use of different intake manifolds and air cleaners, and either finding, or modifying your accessory brackets to work.  The guy I talked to about this swap said he took the stock 307 brackets and basically stretched them (cut them and welding material in the middle) to fit the big block.  Other options would be to mix and match old and new brackets until you get something that works.

Good luck!  

I did a similar swap years ago, putting a 455 olds in a 79 cutlass body in place of the CRAPPY olds 260 V-8. It was extremely easy and it was about the smoothest non-stock swap I've ever done. I did however, grab the frame mount plates from the donor car... in my case, a big Delta 88. I bolted on the brackets and dropped in the 455/TH400. The only real close area was the passenger rear valve cover, where it meets the A/C firewall box. BTW.. it is best to have the new motor as complete as possible, as far as brackets go, but, if I remember right (this was 20 years ago...) either the small block brackets fit or were double drilled to work. Of course you could likely re-use the big block brackets as well. I had to flip the mounting tab to the opposite side of the rear x-mem. to fit the TH400, but in all it was easy and man, did it seriously kick some hind-quarters on the street! I could still get to the plugs and clearance was great! I decided to keep the stock manifolds... with a motor that big, I felt headers were not worth the PITA factor. The weakest thing on that car was the junk one-wheeler 7.5" and airplane gears... In all it was a great swap and I'd do it again if I came across the pieces to do it.