Hi all, I’ve been lurking on this board for a while but I want to chime in here since I have tinkered with efi on the Nissan straight sixes, and I have been keeping it in mind for my M37 as well.
First off, with these engines being what they are I certainly wouldn’t worry about the crank flexing to any degree. That’s generally a problem with high RPM harmonics in race engines, and it wouldn’t be a timing issue. It would be an issue with main bearings and the crank breaking. I have triggered an L6 with a 36-1 trigger wheel from the crank snout with great success, that was hitting easily twice the RPM these flatheads top out at.
When I did my conversion, I found you needed to have a very rigid bracket for the VR sensor or any shake would cause issues with loss of signal. Similarly the missing tooth wheel had to have very little runout as well.
Secondly, I think injecting the fuel just after the throttle body is the way to go to get around the issues with Siamese port engines. Generally with siamese ports you have to do some very odd timing to get port injection to work, and end up running into other problems as well, such as very short injection pulses.
Another thing to remember is that the latest generation of injectors is miles better than the TBI injectors of the 80s-90s, and you can run very large injectors (1000cc or more) with great driveability using megasquirt/ micro squirt.
Anyway, I think these engines would certainly benefit from some form of modern efi, and a coil on plug ignition, or wasted spark using a coil pack.
Tuning is not difficult either, although it does take some time simply driving in different conditions and tinkering as you go.
I say go for it! I plan on it down the road as well.
My 2 cents.