Honker II was like a lot of non McClaren cars needed development and testing, Dan Gurney was in a similar situation with a ford engine with his weslake heads.
Looking for example at the 66 Laguna Seca race 40 cars were entered, 11 were Ford powered and one each by Olds, porsche and ferrari. The rest were Chevs of various sizes and configuration.
The Porsche 917 was a multicam engine, so I cannot see the use of the Repco 769 being a rules hold up. I can, however, see it being somewhat underpowered against 500-plus cubic inch big block Chevrolet V8s and turbocharged Porsche Flat-twelves.
One possiblity. Oldsmobile puts it 32 valve Big Block into production. An all aluminum version is in the same size range as the BBC. Weight should be comparable. But it wasn't just the engines. MacLaren simply had a better chassis through 1970
MacLaren's secert weapon was Bruce hinself. The one US combo that could have competed earlier was Penske. But they wre pre-occupied with Trans-Am
http://www.streetmusclemag.com/news/the-w43-oldsmobiles-dohc-455-v8-that-never-was/