OK I think I get it now.
You could lengthen the (too short) Fast Guns with a section of chain* - Thats what I do as I bought two of the short ones in error back in 2009. I had HJ bumper buttons in the rear on our old truck. Found out, the short Fast Guns are for the front tiedowns. We have TL tiedowns all round on the new truck.
Do you have four short Fast Guns?
You could try to sell both sets and buy the correct length Fast Guns.
Or you could probably try to combine both sets, as you described in your question. The Fast Guns disassemble easily, so you should be to find out how long the hook section needs to be / where the nut is that it bolts into inside the housing. At least I think I remember that mine came dissembled from the factory. Unscrew the hook part until it comes loose, then the retainer nut is free inside the housing. There is a plastic cap on the upper end of the housing that you can remove. The loose retaining nut just comes out at the top end. At least I think that is how mine work. I have never used the standard turnbuckles, but I imagine they disassemble just as easily.
* if you do this, you need chain with welded links and one threaded chain link for each tiedown. At the time, I think TL had included the chain/link hardware with the tiedown kit.