I wouldn't do this job without pulling the engine. On the other hand, if you don't, it's still possible to change the main-bearing inserts with the engine and crankshaft in place providing you can remove the pan.
The bearing-insert halves in the caps are no problem to change, however you'll have to "roll" out the top inserts. This can be done with a special tool inserted in the main
oil hole, a tool that looks similar to a nail. Guess you could use a modified nail with the head bent to lay flat on the main journal.
With this tool in place--doing one main at a time--turn the crank so the nail head butts against the end of the insert that's opposite the tang end. Make sure the head of the nail is low enough that it clears the block as it turns.
Reinstall the new insert in a similar manner after inspecting the main and oiling the insert. Reinstall the cap with a new bearing insert, torque the cap bolts and go to the next main.
Don't forget the rear seal while you're at it. Hope it's not a rope seal, which are hammered into place. Chances are it's a split lip seal, so this job won't be too bad. On the other hand, if the rear-main seal is the later full-circle type, the transmission, clutch and flywheel will have to come out.
Good luck. This will be a neck-bending job, but it's possible.