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Discussion Starter #1
Looking to replace the rear frame rails on my 70 Nova, the quarters and outer wheels houses are being replaced as well, I would like to get the rails done before doing the rest. Only question popping in my head is how the body gets supported or secured to the Comp rails. Has anyone used these before and have photos, or does someone recommend a different rail? I dont want to go with the stock sheet metal rails, because eventually I will need more tire room.
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66 Chevy II, Pontiac powered; 68 &75 Firebirds
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I searched for these:Competition Frame Rails C3055, and could not find anything, so I don't know what they look like. All Novas are a unibody construction with a spot welded sheet metal rear subframe. On my 66, I drilled out the spot welds to remove the bad original rails. I replaced them with a set from Autoweld. The contour lined up well with contour of the car. I plug welded them in from the top after drilling holes through the sheet metal. Other frame rails that might not follow the contour would need to have sheet metal welded to the rails then tied in to the outer body. (I.E. wheel wells welded to the frame and the quarter panels.) I would suggest getting a rail that matches the contour of the floor. You can place them where you want to accommodate wider wheel wells. The only difference between my 66 and your 70 would be floor contour and basic length/width dimensions.
 

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Competition Engineering frame rails are designed to be used with their weld in subframe connectors and laid out parallel to each other. It sounds to me like you’re looking to do more of back half type of build out on the rear of your car. That too would in most cases put the frame rails in a parallel orientation. Typically, back halfing usually prioritizes structural reinforcement over factory floor sheet metal preservation. I believe it is possible to do both but you have to build around the factory floor contours and that can be challenging. I am using a set of Competition Engeneering frame rails in just this fashion on a 1st gen Nova. They will allow me to maintain my factory floor and structurally reinforce my car. They are sort of a generic frame rail shape that is in the ballpark contour wise but not an exact match to my trunk floor. I have had to modify them a bit to make them work within my concept. So far I’m happy with the fitment I have been able to achieve. I’m building a full frame that will be a bolt in and not require any major modifications to my factory floor.
 

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Discussion Starter #4
Yes it will be more of a back half but retain the rear seat. I think I had the mart number wrong there C3035 rails. They are parallel in nature, so far I have the subframe up and mounted with solid bushings, next I will run a drop crossmember between the rear most rocker boxes then use some square or rectangle stock to weld from the drop crossmember to the front sub frame. My question pertains to the rear section from the crossmember back. I know that I will be running them inward 4” per side from the factory rail location.

I guess to simplify the question, can I run the new parallel rails from the drop crossmember to the rear crossmember under the tail panel build a new sheet metal floor in the trunk and call it a day? Or do these rear rails need to be tied into the body in more places and if so where? Or will the car need a roll bar with rear bars at least.
 

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I’m not that familiar with 3rd gen Nova’s but generally speaking the rails should be tied into the unibody structure in significant ways for it to be any good. If your going to transfer the suspension loads from the factory points to your new narrower subframe it will have to make up for what ever you are removing. I would look at back half kits that are available on the market to get an idea of how they are tied in to the car and model your subframe after something that looks like the suspension set that you want to run in your car.

I would suggest looking at Chassiworks, Art Morrison, and Autoweld for back half clips to study for your design concept..
 
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