The hard part about "tuned length" pipes is the "tuned" part. This usually means that they are made to kick in at about the time that the engine is reaching it's peak output. You need to know about where in the rev range you want them to work their best.
I presume that if you ring around the better quality exhaust shops, & reputable Dyno facility or two, & / or some of the more successfull racers, you will come up with the info that you need.

The genuine "tuned length" pipes will likly need to be custom made for your application i think.

A similar looking header that might suit your needs might just be the "equal length" headers. These aren't tuned, but look & work well anyway. Still likely to be a custom made job i think. My Tri Y's were custom made & it was no big drama.