I have an 87 Custom Cruiser with a 307 and I have a lead on a donor car with a 403. I have heard the blocks are the same size, but what should the casting numbers on a 403 be? I am interested in makeing this swap if it won't prevent me from keeping my A/C or overdrive trans. If anyone has any info on this swap I would appreciate your help. TIA

 

The 307 and 403 (and all other SBO motors from 1964-1990) are externally identical. This is a bolt-in swap. Everything from the 307 bolts directly to the 403, including the accessory brackets. You need to be sure that the water pump, pulleys, and accessory brackets are used as a matched set (ie, either all from the 307 or all from the 403). If you plan to retain the R4 A/C compressor, then you'll need to use the 307 hardware.

A 403 block will have a large "403" cast on either side of the block, above the oil pan rails.

You need to decide if you plan to retain the CCC system or not. If you are subject to emissions testing where you live, then you'll need to retain the system. The CCC carb can be made to feed the 403 with some simple mods (richer secondary metering rods and filing the stop on the secondary air valve to allow full opening). The one downside to the CCC system is the inability to alter the timing advance curve without burning a new chip. Alternately you can ditch the computer and run the 403 carb and distributor. You will need to come up with an external means of operating the TCC solenoid and the A/C compressor clutch relay if you do.