those in search of "high end" 904 parts you might try and contact "David Dean" over at Moparts.... he's broken about anything you can break on a 904 and is moving on to a powerglide, so he may have some parts of interest.... and that was with his old mill. Believe he's gearing up for his W8/9 headed monster....
I have a 904 that has been rollered, lightened and sports a Tranzact non low band apply trans brake. We ran this behind our iron head 360. When we "stepped" up to our W5 project, I opted for the 727 as it was obviously the more "reliable" of the two and my dad was racing it at the time and felt it would be a better "set it....and forget it" choice. I'll be running that also with the W8 mill.
Pro Trans..... If I'm in Dave's shoes would "I" want to sell a "warmed up" 904 to a customer hitting it with drag radials and a shot of NOS here and there??.... uhhhh probably not. You build your trans with upgraded items here and there, and it breaks..... it's on you. You buy the same trans from Dave, break it..... you've broken your PRO TRANS.... Dave sells some pretty nice pieces and when you step up to the plate, you are doing so for reliability.... same as Chuck Lofgren who originally built my big block 727 and our race 904. I had the pleasure of meeting Dave a number of years ago. My trans had started to creep when the brake was applied. I had no idea who he was, but some west coast guys recommended him. Tore my trans down, boxed it up and met him at his shop one winter morning. We did a quick walk through what I had, he pointed out the issue, I had him add a rear roller support and machine my front drum for better sealing rings. Worked like a champ when he was done. He and his son Darren are good people in my book.
