PA registration is a rough road.
I went through it 2 years ago and there have been some changes in the law since then. At first I thought that if I pay all the sales tax (for the parts purchased out-of-State) how hard could it be? It's hard but it was worth it.
You really need to be prepared for some very challenging problems during the build. PA registration, compared to some things, was not in the top ten.
PA has a specific category detailed in The Pennsylvania Code subchapter K. Street rods, specially constructed and reconstructed vehicles.
This Street Rod registration permits many things you cannot do otherwise, such as a chopped (7") windshield, no bumpers and no fenders.
Try to find a Notary that handles Street Rod registration. The Notary (Ann Marie Mint) in Coopersburg is geart. And an inspection station that handles Street Rods. Wes Jackson is Royersford is the best. Your local State Police barracks can give you a list of inspection stations that do Street Rods. These people know what it takes and keep up with any changes.
SAVE YOUR RECEIPTS no matter what State you purchased the item. You will need to prove that you paid PA tax for everything. Any unpaid tax will be due at the time you apply for registration.
What are you planning to build?