You are exactly right. I have chosen a 3" x 4" HSS that I will modify to add a sloping surface of 17 deg for the firewall, it will end up being 3" x 3 1/2" approx.

I haven't really decided how to tie the transverse frame to the upper frame rail. At the moment they just sort of intersect as shown.




I will have a 1 1/2" x 3" HSS connecting the transverse frame to the lower frame rail so perhaps I will design it to tie all three points together.
I also plan to put a large gusset on each side to tie the transverse frame, lower frame rail and firewall together.

edit: for typo's

[This message has been edited by Yarmouth Fiero (edited 01-25-2013).]

Perhaps extend the transverse frame like this, weld it inside and out where possible and then cap it with a piece of plate.

Just a thought.




edit: to add a view

[This message has been edited by Yarmouth Fiero (edited 01-25-2013).]

Thanks Don. As I have it drawn at this time, the firewall top edge will remain stock height. If I had a rear window, the frame would certainly be too high as the window ledge in stock position is about flush with the top of the upper frame rail.

To answer Blooz, no, there is no real reason to raise the frame up to the very top of the firewall, other than to gain that 1.44" in the engine bay. Well, also, that "Z" piece of sheet metal that helps support the rear window and edge of the firewall has a flange pointing down against the firewall so even though its only 1/16" thick, it would keep the transverse frame from laying flat against the firewall.

[This message has been edited by Yarmouth Fiero (edited 01-25-2013).]

quote Originally posted by Yarmouth Fiero: I imagine Blooz that we also must keep in mind that the 355 body ( rear quarters and engine deck) are going to be almost flush with the top of this transverse beam, atleast in my current configuration, so as the rear quarters come around and over the B pillar, they will start sloping downwards, possibly right over the upper frame rail so we'll have to keep the top of the transverse frame as low as possible on the ends.

As long as you stay only as high as the the top surface of the firewall, you should be safe. These two photos are the best I could do to show the relationship between the underside of the rear fenders and the top of the upper frame rail. Even though you can't see the upper frame rail, it is 25mm below the top surface of the firewall, so you should be able to visualize where it is in relation to the fiberglass. As you mentioned, if you allow your new crossmember to extend over top of the upper frame rail, you'll probably need to bevel the ends:





As for where the decklid sits in relation to the top of the firewall, here a couple photos that show more or less what you want, though when I disassemble the body to modify it, I'll have to change this area somewhat since the thin lower fiberglass window frame isn't supported by anything as it is installed right now. I will probably fabricate a metal window channel that is welded to the frame of the car and have the fiberglass simply act as a decorative cover. Even though you won't have a rear glass window, you'll still need a trim panel in the same area so you will be affected by this too. The first photo is just to orient you:





Currently, there is approximately 65mm between the upper surface of the decklid and the top of the weld flange on the OEM cross-beam, but like I said, I'm not far enough along to say for certain whether this should stay that high or not. You don't have to look closely to see that the lower rear window frame isn't straight so take the 65mm with a grain of salt. The 30mm dimension shows the location of the underside of the decklid in relation to the upper side.

As for how much room there is between the underside of the stock cross-beam and a typical SBC installation, it's tough to measure because of the angle of the valve covers, but here are a couple photos that show you've got lots of room:





Bear in mind that this is an '84 chassis so the wire bundles come out in a different location than yours will. That's all easily modified anyways.

[This message has been edited by Bloozberry (edited 01-25-2013).]

Thanks Blooz. That is very helpful. I guess I could just remove the 'Z' piece which would move the transverse frame up flush with the top of the upper frame rail and it would allow the transverse frame to sit tight against the back of the firewall.

BTW... PIP is very slow tonight.

I'm going to add the stock engine deck hinges to the drawing so see where the engine deck will be in relation to the top of the firewall. I suspect the engine deck height will be very close to the stock Fiero?

[This message has been edited by Yarmouth Fiero (edited 01-25-2013).]