I'm rebuilding a GM 2004R for a guy down the street from me for his 69 Camaro. They are a good trans if you put quite a bit of money in them. One of the biggest costs is a billet or hardened forward drum. They can be built to handle 1000 HP according to Art Carr but that takes some doing (money). IMO, if you have something making over 400 hp by the time you buy a 2004R core and build it to withstand the power your throwing at it and buy the adapter to make it work your better off with a A500. The only disadvantage to the A500 is you have to massage the trans tunnel to get it in. In both cases (2004R vs. A500) you will be modifying the TB crossmember. JMHO