Many times, header fitment is controlled by the engine location. Aftermarket headers are usually designed to fit fine, providing that the engine is in the stock (or OEM) location in the chassis. This means that the chassis and the K-Member and the engine mounts are the same as OEM, or very close. Remember, the headers bolt to the engine and if the engine is not in the stock location, there is always the chance that there will be interfearence between the headers and some part of the vehicle. Most tubular K-Members are not a problem. Some may have more than one mounting hole for the engine mounts, which may permit the engine to be relocated back toward the firewall. Use caution, and make sure that will not cause a fitment issue with your headers. The photo below shows a 302 engine (8.2" deck height) mounted in a FOX chassis on a tubular K-Member (I do not know which brand) and some 1 3/4" RCI headers, bolted to some AFR-205 cylinder heads (using the optional 3" wide flange bolt pattern). Everything fits fine, including the OEM steering shaft. Note that some of the tubes (the #3 tube in this case) goes over the top of the steering shaft. I can't tell what engine mounts are being used, but they obviously are keeping the engine in the correct location. You can see that if the engine had been moved back toward the firewall, or had been lowered in the chassis, there could be interfearence between the #3 primary tube and the steering shaft. High performance headers are designed like this if the customer wants (A) a reasonably equal primary tube length, and/or (B) to have the headers clear an SFI scattershield or a wide automatic transmission (AOD, C-6, Powerglide, Turbo400, etc.). This is where some of the cheaper "one size fits all" production headers have problems. When taking header orders, I always ask the customer what engine mounts he is using. Many are using aftermarket mounts. Alot of the engine mount manufacturers are telling their customers that the mounts are the same height as the OEM mounts, even when they are not. I have seen some that lower the engine up to 3/4 of an inch. This may not be an issue when running shorty headers, but could cause an issue with long tube headers (as well as ground clearence). If this happens, it's an easy fix. Just add some spacers (or flat washers) between the engine mounts and the engine block. Just things to think about...