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How large is "large"? There have been various comparisons of linear rail vs rod and the rods tend to compare very well. Have a look on YouTube - there are some good examples where people have swapped out rods for rails on established designs and done before/after print comparisons.

Where rails come into their own is when the weight of the printer would cause the rods to sag as the rods have to support the weight as well as act as the guide. Rails are supported so you have far fewer issues.

From what you are describing, it sounds like you will have a potentially very heavy moving mass on the head end. This will cause you lots of problems with speed and accuracy. Are you sure this is the route you want to take, as the general approach is to minimise the mass of the head end as much as possible. If the head end gets very heavy, you may want to consider a threaded rod rather than belt approach. That comes with its own problems (which people will tell you about in no uncertain terms ;-) ) but they will handle a heavy head end much better. Expect threaded rods rather than belts to be much noisier and slower...

If you know all the above then my apologies... but please share your progress as it will be an interesting build!