you have not mentioned what gearbox you have ?

the 5K will get you nothing if you havnt already got a box to handle 500hp nevermind 600hp

with the rear diff, you only have 2 real options, stick with ford and use the 9" ( there goes most of your 5k ) or use a non ford item for considerably less,
the front fit the 8.5" group a diff sump and crossshaft ( 4.5K)

these are your only real guranteed reliability front and rear diffs, all others are just holding in there waiting to break, you get what you pay for.

all use dependant of course, if you just go shopping with your 600hp and never come on boost then just stick to std