it sounds like you don't want to do the rails and you are looking for an approval from somebody in the forum...and I do not blame you for it. What people are telling you here is that if a piece of the rail broke off with a tap from the head, chances are, that the rest of the rail is just as brittle and may break in the future. You are already half way there so.... Bad things can happen to your engine with a broken rail but, if you take the risk and decide not to change the rails at this time may be nothing will happen but, may be...
Bottom line, this is your engine, car and money. Also take in consideration how long you are planning to keep the car, the condition of the rest of the car and if you have the extra time ( a lot, depending on you expertise), tools (not many) and manuals (highly recommended) in doing the extra work. Only you can decide...
Good luck with whatever you decide to do!

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J.H.
'86 300E