* Transmission: The biggest concern is whether your existing transmission (and bellhousing) is compatible with both engines. While both engines are small-blocks, there *might* be slight differences in the bellhousing bolt pattern depending on the year of the engine and transmission. You'll need to verify compatibility. If the bellhousing doesn't match, you'll need to swap that as well.
* Mechanical Clutch: The mechanical clutch itself is likely unrelated to the engine swap. The clutch system (pressure plate, clutch disc, throwout bearing, etc.) is attached to the transmission, not the engine. As long as your transmission remains the same, the clutch should work fine.
* Mounts: The engine mounts might need to be adjusted or replaced; the 350 and 305, while similar, are not identical in dimensions.
* Wiring and Accessories: While wiring harnesses are largely similar, there might be minor differences, particularly if the engine is from a different year model. You'll need to check that all sensors and wiring are compatible.
In short: The engine swap is *possible*, but requires careful verification of compatibility between the 305, your existing transmission, bellhousing, and engine mounts. Consult a service manual specific to your vehicle's year and model to be absolutely sure of compatibility. You might need to source additional parts. Assuming everything lines up, the mechanical clutch aspect should not present a significant obstacle, provided it's appropriate for the transmission you are using.
