Beware: I put an engine from a 404 in my 1951 C to get the manufacturer recommended 35 PTO HP to pull a finish mower. My decision was based on advice from this site. My options were rebuild and power up the weak super C engine already in my Grandfathers tractor or replace its engine. My thinking was that for the long haul the 404 hold hold up better ( more modern design, 14 bolt head instead of 9 bolt, etc.). After using it for 1 season I wished I had gone the other way. If you are considering it here is some things to consider. 404 is rated at 36 PTO HP at 2300 RPM , for my tractor PTO to run 540 RPMs it's engine must be run at 1875 RPMs, it bolted up at the transmission and front end but neither 404 or C radiator would work, I bought a radiator from 140 but it does not cool effectively and the engine runs on the hot side. The C governor would not work so I had to buy the 404 governor. The generator would not work so I found and purchased a 6-volt positive ground alternator. The hydraulic lines would not work so I bought a pipe threader cut the lines and put in flexible lines. The 404 had more gauges than the C so I had to buy mechanical gauges that would work on the C. The front Grill will not work so I am running without it. If Grandpa could see it now he would roll over in his grave. These next issues are not a result of the switch but a poor past mechanic but it could happen to you unless you know the past owner. After hearing it run before I bought it I thought it sounded OK. After getting it home and giving it the once over I found a ground up distributor/governor drive bushing, timing gears 1 tooth off, bent push rods, a bad oil pump, stripped out oil pan bolts and roofing cement as an oil pan gasket. So beware if you are considering this I did enjoy the challenge but it ended being 4 times the money I had expected and a lot of extra time. GOOD LUCK!!!