[PNJSNAK]

rdc;
I have a 351 W. I mounted the headers on the motor, then tried to lower the body down. At first it would not go but we tipped one side up and the other side down and it went down to the frame.
You may want to put the body on first then mount the headers.
Which ever way you go, once the body is down on the frame and positioned the way you want it. Drill some pilot holes thru the body and the frame and put in some of the big pop rivets as guides only, do not use the pop rivet tool at this time. These holes are only to be used as pilot guide holes to reposition the body back on the frame, because you will want to remove the body to do a lot of the remaining work with the body off the frame.
Now that you have the body positioned on the frame with the pop rivets as guides and the headers mounted on the heads of the engines. You can determine with a straight edge where the side pipes are going to go thru the body. Once you determine where the opening will be, mask off the area and use an air operated little sawsall (best tool I ever bought) to cut out the opening. I tried to get at least a 1/2" clearance around the pipes at the exit area of the body.
When you are doing this be very careful on where the inside body panels are for the engine compartment. I was very close to this panel when I began to carve out the opening.
Once you have the opening cut out you can go back and make the final opening with nice blended generous radii with a sanding tool drum.
Make sure that when you position the side pipes on the headers you maintain at least 3/4" to an 1" of clearance along the body for the side pipe. Determine where you support mounting bracket is going to be at this time. I welded a small angle bracket on the side pipe and mounted a bracket off the frame that I bolted thru the angle bracket to the frame support with a rubber bushing for vibration.
What I learned and followed thru all of my build was I thought about how I was going to do something and then thought about what it would look like and then thought about it again, then made a sketch and thought about it again then went and did it.
I wished I found this site before I began.
Wait until you make your first cut into the body, it is a real "Sphincter" (sp)tightening moment.
Jim