Just a thought here . Are we certain its a sm420 , in some of the bigger trucks 1st and 2nd were in the normal position but 3rd was in the 4th gear location and 4 th was in the 3rd location . the reason was 3rd was direct drive and 4th was an o/d . this could explain the unwillingness to enter the next gear as its could actually be dropping a gear . If the trans. had been changed it might not match the tag on the dash for shift patterns .
John
When I pulled the trans, all the numbers matched that of a SM420, and the guy I have rebuild it indicated that he had to source pretty far and wide for the needed 420 parts. In the 400 or so miles I have put on her thus far, 4th has got to be the direct drive gear. 3 hi wound out on the engine produces about 25 mph, and I haven't even tried to push 4 hi beyond the 55 mark. 55 is about all she wrote.
Also, The trans shifts perfectly between 3-4, its just that 3 hi to 4 lo axle shift that's wonky
