we have done several big block conversions but none using the factory transmission which gives one no leeway for off setting the engine (steering box) or moving forward (firewall clearance). I would think the small distributor on the BB will clear but almost sure you will have to do a bit of BFH on the firewall for the rear of the right cylinder head. Small block 400's are getting hard to find but do have a lot more torque than the others. Here is a shot of a 1937 2 ton with a big block and a bubble for right head clearance---ignore the big bubble for an HEI distributor and look at the small one to the bottom right if the distributor one. This truck also has a shorty steering column with a U-jointed shaft to clear exhaust manifold; something you shouldn't need.

[img]https://i.postimg.cc/v4nQYK2x/37-Hauler-Don-s-for-sale-86-006.jpg[/img]

[img]https://i.postimg.cc/3WkzQvcd/37-Hauler-Don-s-for-sale-86-005.jpg[/img]

[img]https://i.postimg.cc/N5hChPXz/37-Hauler-Don-s-for-sale-86-007.jpg[/img]

Evan