64 C20 engine and tranny install

[Desert dog]

My first post!
As mentioned in the welcome section, I'm helping a friend revive his truck. He has a '64 C20 and is wanting to install an '86 305 engine to mate with his original M420 tranny.
After some research we are planning to use an '86 168 tooth flywheel along with a '64 11" clutch kit' and '64 bell mounted starter. The engine was originally mated to an auto tranny so a pilot bushing will need to be installed as well.
The whole arrangement has been sitting out for a while so a cleanup of the crankshaft mounting and bolt holes will be in order.
I am debating as to whether to pull the tranny and bellhousing and mate everything together prior to installing them, or as he wants, try to stab the engine onto the tranny in place.
I would appreciate any feedback you would be willing to provide.

Regards,
Desert dog