Chip,
going by the book on the older 235/216, it is possible to lower the crankshaft by about 1/2" to 9/16". This is a procedure used to replace the main bearings. This also means removing the transmission and front cover.This would allow you to flush the block out usuing the oil pump and solvent.
Flushing: I would be flushing with solvent using a new oil pump while the motor is apart. The top can be flushed by pouring on the head. This will ultimately run through the side cover area so it should come off. You can also rotate the crankshaft and blow in compressed air into each main journal hole. The truth is though that to be sure, a complete disassembly is required. You probably would not have to buy a whole lot of new parts, just gaskets.
