A picture of what you are looking to do and/or the house might help us all answer your question in a more helpful manner.
I know it's not always possible to do a pic but there's so much to consider when you are doing a porch on a old home.

I've uploaded the picture to the gallery here, which hopefully will work. This is my first time doing this on this site, so I've got my fingers crossed.

The house has three layers of "siding" on it, with vinyl on the outside and old cedar shingles on the most inner layer. We are putting clapboard on the house now.

Thanks!

Here is a link that might be useful: Construction photo

We did ,we got it from our guys that did the restoration of our home.

How much fir do you need for the interior renovations? If not a huge amount, you could cut it out of the inside of closets and then use the oak as replacement wood into the closets? If you need a full room's worth, then you could do a decorative wood transition strip/pattern and change to oak from that point on. Just a couple of ideas.

For the porch,

Unfortunately we need more than the closets etc. will provide. I think your plan for the wood transition is a good one and we'll have to sit down with some paper and figure something out. The cost of reclaimed wood is just too high here and its something we can't swing. I was hoping for your porch thoughts, but alas they seem to have been cut off-