I've worked on quite a few 4BT's using the old T19 transmissions. They used the 11" clutch/ bell housing set-up common to the 70's-90's Ford pick-em-ups. All the 6BT's I've worked on used the Clark/Eaton 5 speeds. Good tranny's but Lordy, they were HEAVY!

It oughta be quite the project.

Have you considered the 4BT engine instead?

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now