In no way would I expect the dovetails to be flat or straight. Infact it is almost a given that they will be out a few thou somewhere along there travel..
This is enough to cause major binding in a good preloaded rail system once installed.
IMHO making a new gib and scraping it to fit the system would be well worth the effort of learning how to do it. A small surface plate would be all thats needed for a gib of this size and its a skill that will help you with many other projects down the road. Also the connection of the head to the slide where it rotates can be a problem which can be from not being flat as it should be, or the screw nut connection having problems which is documented in many of the RF-45 CNC changeovers. You would be surprized at just how much a good gib fitted to a machine will help. Infact one that doesnt fit good will not even ajust right on the Z axis and leave you with slack.
By all means look at the small things first before going down a major mod road such as adding rails. Rails are good in many ways, but they must be done right.
Not trying to talk you out of adding rails, but aproaching the problem with some determination and looking into the complete system may save you some grief.
If you do go with rails prepair the mating surface or you will have problems.
Jess
GOD Bless, and prayers for all.