Re: ProCo RAT into my JCM 900
it will sound good, but it wont sound like an 800. I used to use a dual reverb 900 back in the early/mid 90s and i played in a band who used the 800/rat combination so it was a pretty clear comparison. His 800 was very dirty, so hed only use the rat for leads.
However, i still have a jcm900 master volume, as well as a bunch of od pedals including a rat. I tend to run that amp low gain and really like the mxr od for its dirt. sounds titanic. With the rat its nice too.
So - for your amp, i reckon you will get a really good sound this way....
Set up your clean channel how you like it, then use the rat for your crunch sound.
Then just set the lead channel up with a hair more gain than the clean channel and you get 4 useable (and switchable) sounds.
Rats are very good for an 80s rock tone and can do a pretty good marshall imitation thru a fender too. Their eq is very similar to a hotrodded 800. I can understand why you want the whiteface reissue, they are good like the old ones, but for less bucks and a little more versatility you should also check out these:
http://www.hartmanelectronicstore.com/LM308-Op-Amp-Distortion-OAD.htm
really good pedals, built by a total gentleman.
FWIW - easily my fave od pedal with my 900 is the MXR gtod (or classic overdrive from gc) which is not expensive, but somehow sounds better and hairier than all my other overdrives/distortions, so if you can score one cheap then its definitely worth looking at.