I really have nothing in common with your set-up other than it's an
a-body, I'm running a shield and I had collector clearance issues.
First, let me start by saying that your endeavour will require some fabrication whichever route you choose to go. I personally would run a shield and not a blanket for a couple of reasons. The first being that the shield does not come down past the pan rail and really shouldn't affect header clearance. The second is that the blankets (I've run one before) get real nasty after a while, especially if it's a street strip vehicle, and like to absorb oil, tranny fluid, cooland and whatever else may find it's way into the tunnel. That being said, I don't know too many cars that can run one without tunnel modification, so there's fab work involved.
Should you decide to run a blanket and don't want the "wick" it becomes when soaked with oil and what not to catch fire, you can cut all of the primaries at the vertical point just above where they bend 90 degrees to run horizontal and parallel to the floor, rotate them outward and re-weld. Again, requiring fabrication. Depending on how close you really are and how much clearance you need, you might get away with heating the collector cherry red and bending it outward slightly with a pipe. Bear in mind that there's a fine line between a slight bend and a crimped collector!
I've attached some pics of the wacky collector arrangement I was dealing with and a mock-up of the tranny with shield in the car. I modified the tunnel and the collectors for what it's worth. I hope this helps!
Oh, and yes, it's a GM transmission which forced me to cut the floor anyway, but the collectors wouldn't have cleared the pan on anything!
