None of the 300's or 350's had the sideways engines, except for the 350 Rancher, but I think he is talking about the Fourtrax 350 or 350 Foreman. If I remember right, the first year of the 350, it was called Fourtrax, but the following year they changed the name to Foreman. The 300 has a transfer case, but a 350 don't. The carbs are different, but might could be made to work. The gas tank is completely different. The 350 has the gas tank under the seat and uses a fuel pump. The 300 has the tank in the normal place in front of the seat. The 350 has a solid front axle and the 300 has IFS with A-arms, unless it's an '88-'92 2 wheel drive, in which case it would have struts and lower A-arms only. Rear hubs are different and I believe front hubs are different too. I can't really think of anything that is the same.