Here's an article on how Ford developed the 427 SOHC in only 3 months in an effort to compete with the 426 HEMI.
Other than problems with the 7 foot long timing (cam) chain stretching, (there may have been other issues, but the chain stretch is the only one I am immediately aware of) the 427 SOHC was an engine that did and should hold a rightful place as legendary.
https://www.goldeagle.com/tips-tools/the-cammer-fords-answer-to-the-hemi/
So no, as I suspected, Chrysler didn't "help" Ford develop the SOHC 427, and I'm sure (without any research) the same holds true for the Boss 429.
I've seen them spin over 7k RPMs and that is not unusual, but "normal".
No don't get me wrong, there is NOTHING, including the awesome GEN 3 "HEMI" that tops the engineering, design, and performance as far as I'm concerned (especially when built up beyond stock specifications) of the 426 HEMI. Sure, the chiller system, supercharger, EFI, and computer system of the new GEN 3 HEMIs are outstanding engineering, but for its time, and still today, the brain power that developed the 426 HEMI is second to none.
The Boss 429 was worse for being "tempremental" (read that as "ran like crap unless tuned to a T" ) on the street in stock trim than the HEMI, but the performance of the Boss 429 was also outstanding, especially in race trim, and in some cases, possibly due to its larger intake valve diameter, superior to a comparably built 426 HEMI on the top end or higher RPM range.
Still, when any of the "classic" engines of the muscle car era are taken to their fullest potential, I don't think there is anything that can out power a GEN 2 HEMI.
Last note: although GM had higher horsepower output on their high end big blocks than the
440+6bbl, those numbers also came in because of higher compression, solid lifters, and aluminum cylinder heads from the factory. I am proud to have one of the most powerful big block non-HEMI engines produced by Chrysler, and like I said regarding "worth the difference", the difference between the experience of hammering my 440 4bbl in my Charger R/T back in 1978 vs my warmed over 440+6bbl in my Roadrunner is night and day. Other than the feeling I've had when boost kicks in from a turbocharger, there's nothing I've
personally experienced that compares to the outboard carbs opening on my 440+6...it's a RUSH!