For the chassis look at the pictures. 400 clutch pedal has the bolt and lock nut. 450 doesn't have the hole. Also notice the 450 from the pivot point to clutch rod pin hole is a shorter distance. If it has a T/A, the levers in the picture for its operation will have the first two numbers on it starting with 35 for 400 and 36 for a 450. Also notice the 450 has a shorter side than a 400 or SMTA. If they have original engines. All engine serial numbers for a 400 will have 6 numbers. 450 from 3 to 5 numbers. Engine number is stamped in below the # 3 spark plug area on a raised flat. Month and year the engine was installed can be determined from it, and if original will tell the month tractor was built. Original 400 block has C-264 cast above the oil filter. 450 has C-281. But if the block was replaced sometime that could end up different. Real late 400 tractors had the hole for hydraulic valve selection in the instrument panel shroud. Never saw one with the 450 keyhole opening though unless it was a replacement part. 450 also has a slot and hole in the steering support casting for valve selection.