ADR27A is all external stuff other than the heads so as long as you use the heads and all the pollution equipment it shouldn't be an issue.

For the same timeframe build the 11QT and QT engines are the same. But from HJ the piston dish decreased to increase compression from 9.0:1 to 9.7:1 making the HJ a significantly more powerful engine, so much so that there was no longer any need for the 350 and 400 proposed for HJ espeially with a dual exhaust system.
Then ADR27A hit in HX and the manifold fitted plus the tuning of the engine basically killed it. People knew this from the HJ to HX transition, but imagine having test drove an LX SLR or SS with the HJ engine which with its proper 2" twin system and light weight was a very fast car (as quick as or quicker than a stock L34), so you ordered one. Then this puking, gutless post 6/76 one turns up. You'd be spitting chips!