I think the short answer is: just bolt all the 3.4\3.8 bits onto the 4.2 including flywheel starter etc. The only caveat 'may" be that the flywheel is not balanced for the 4.2, not as imbalance per se, merely that I think the 4.2 flywheel is heavier. which may have had some impact on the generally less smooth operation of a 4.2 as compared to 3.4, 3.8. But, bottom line is, if the bits fitted together and worked on a 3.4, they will do so on a 4.2, I believe. Edit, 4.2 uses a different front damper, whether that’s compatible with the earlier flywheel, I don’t know, but may be a consideration.
Thanks. The flywheel was about 24# which is heavier than the weight some claimed for a 3.8, but lighter than a 4.2. Heavier than an aluminum 4.2 or 3.8 wheel.