Is "If it had been for you" grammatical?

Yes, it is grammatical. The context in which the sentence appears is helpful:

Farewell, my dear friend; I received a letter from M. de la Mertiniere lately, inclosing a few lines from you; pray thank him for it. I have done all I could for his ode, but he owes me no thanks for this; I acted as if it had been for you. Farewell; thank you again for your verses; I like them, they convince me much more strongly of your friendship than of my own talents.

The meaning here is roughly the same as,

I pretended that I was doing it for your benefit.

The suggested rewrite would not preserve the meaning.

Yes, Cameron is correct, about this construction in this context.

However, you have to distinguish this construction from the NPI idioms

• if it hadn't been for X...
• if it weren't for X...

These refer to a hypothetical outcome that has been averted by X, they require both negation and if, and they are brilliantly ungrammatical without them.

• If it hadn't been/weren't for him, I'd be dead now.
• *If/Since it had been/has been/was for him, I'd be/I'm alive now.

(An asterisk before a sentence indicates an ungrammatical, hypothetical example in linguistics.)