Here's the Ebay seller that offers K7 resin items. As of this writing, there is nothing listed. I heartily second the positive comments of the OP regarding the seller and product.
https://ebay.com/usr/shelbydodge88
Cool! I'm doing something very similar with the Revell '69 Chevelle.
I've gone with the same headers you've chosen. I sanded the frame where it narrows down to provide header clearance.
I understand cost being a concern. You alone know how best to allocate your resources.
I will most likely get some Tamiya compound. I'm most interested in adding value to my scarce building time. Every other Tamiya product I use has added value to my modeling. Less work/better results.
I have a full-size Novus kit that was found in the trash at work. Would you advise it be returned there to be replaced with Tamiya compound? I'll happily spend a few bucks to achieve a better paint finish.
Greg Wann at Holy Grail Resin Parts came to my mind. I've seen small lot/custom cast posts on the Holy Grail Facebook page. Holy Grail Resin has a proboards site if you don't Facebook.
Here's a link to those mustang decals. https://public.fotki.com/RaysKits/ford/mustang/1984-mustang-gt-350/84mustanggt350-20th.html
The best way to contact the gentleman is through his website email.
https://rayskits.com/decals/
I don't have that kit, but I have a set of fugu garage wheels.
The website photos accurately depict the wheels I received. I had no issues with shipping or service, also.
I plan to order from fugu garage in the future.