Welcome, I bought my 1967 back in 1986, pulled the small block and put in a 396, sold it in 1997, down payment on my first house. Wish I never got rid of it, but I'm working on a 1969 now.
C4 IRS in the rear, there's a few that have done it, some 100% custom, some with kits that are not very invasive like a rear clip install. Are you thinking of doing it all by yourself? Fab and install? There are a few shops that sell rear clips for a C4 IRS install. Street Rod Garage comes to mind, their link is listed below. Newman Car Creations comes to mind too.
http://www.streetrodgarage.com/c4vettesuspension.aspx
http://www.newmancarcreations.com/products/
I'm not advocating either (or knocking them either!!), and there are other shops out there doing similar things too. I'd say "buyer beware" as I've heard that there are plenty of shops that show off some of their projects, products and cool things, then don't deliver, or deliver slow and late, over the advertised price. So whatever you do, make sure you check out any vendor before you send off a payment.
Got some pics of the car? And mods that you've done so far?
Is your heart really set on an IRS setup? I though that would be what I wanted too, but after going to swap meets, talking to various vendors and builders, other car owners, etc. I found there are other companies out there that make performance oriented rear suspensions that are extremely comfortable and give just as good a ride without tearing up and modifying the rear of the car.
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
69 corner burner
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