999 clutch pack in a 904' & opinions on fluid

A 999 is a lockup 904 with a rigid wide 2nd gear band, 5 disc high gear drum, lockup pump, and lockup input shaft. So the answer is, no, you cannot put lockup 999 parts in a non lockup 904.

You can however put the 2.74 low gear set from a 998/999 into your 904 if you take the front planetary and sunshell/gear assembly.

I cannot stress this enough regarding 904s--you can get in real trouble really easy by mixing and matching parts. The best thing to do is grab a complete core off the shelf and build it. The easiest way to get 5 high gear clutches is to install five .061" grooved red clutches and five .068" steel plates into your 4 disc 904 high gear drum. Selective snap rings are available to obtain the proper clutch clearance, and you can machine the apply piston in a lathe if needed (I do not recommend machining a step in the pressure plate, and I have never recommended that).