Don't ever rely on your memory for a brake part number. Look it up. Exchange front calipers are available for the 1800. Caliper rebuild kits, master cylinders, brake pads and most other 1970 and later brake parts are available. The 1800 have an independent front suspension nearly identical in design to the P120.
Many part numbers are also shared.
As a general rule, the major difference is coil springs and shock absorbers. Early cars through 1969 have stamped steel wheels 4.5 or 5 inches wide by 15inches in diameter. All later 1800s from 1970 on use the same sizes, but a black spoked alloy wheel was used on a few cars. The steering system on these are a manual cam and roller type identical to the P120 and similar to the 140. Power steering was not offered.