Red motor diff centres will fit into grey motor diff housings using the original grey motor axles.Only thing you will have to change is the rear grey tailshaft universal joint will have to be replaced with a red motor universal joint ( about $30 ) and you will have to use red motor tailshaft universal joint "U" bolts and nuts as well.
Can be done in a few hours or so.
Just have to make sure the red motor diff centre is a course spline centre as they also made a fine spline centre diff in around the HX Holden range and these fine spline centres won't allow the original grey axles to fit.
Red centres came out in 3.90, 3.55, 3.36,3.08 and 2.78 ratios.
3.08 ratios and 2.78 ratios use a longer bolt in them than the other ratios so a little bit more modification is required to fit these two ratios.
You won't gain any advantage with a red 3.90 ratio as its essentially the same as the original 3.89 grey motor centre ratio.
3.55 will give some economy pickup but not a gret deal as its ratio isn't too far from 3.89.
3.36 is a good ratio especially if you are running a grey as it gives about the best economy with a grey without too much loss in pickup/power.
Some blokes have run 3.08 with a grey but this ratio with a grey would probably be best if you do a lot of highway cruising and not worried about power or pickup too much.
2.78 is probably too tall a ratio for a grey I reckon.
Oh , and if you did buy that complete HR drum to drum assembly on evilbay and only wanted the 3.55 diff centre then you may find a willing buyer here ( me ) for the rest of the HR housing ( you keep the diff centre )and brake assembly if that helps you out with the purchase price..... if you were eventual winning bidder that is.
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