1978 cutlass supreme brougham motor swap

[olds307 and 403]

Thanks for the reply, I’m convinced with the 403 motor I’m looking for one online at the moment.. so if I do get my hands on a 403 block would it be able to mate to my transmission I’m not sure what transmission I have but I know have the v8 4.3 litter 206 block and also would I need to change out the brackets as well?

The TH200 trans will bolt up but will probably die in short order. All the brackets swap right over. Basically all the 260 through 403 are the same motor with different size bores and heads. Olds changed displacement on them by increasing the bore. Your 260 has peanut port heads and a 3.5" bore. The 403 heads also suck on the exhaust and have huge 84+cc chambers. Which is why people put on early 350 68cc chamber heads with a better exhaust side. So 9 to 1 compression and better flow. Most also add W31 2"/1.625" valves with bowls opened at the same time along with a cam swap. The 403's huge 4.351" bore means much better airflow. Even the 350's 4.057" is a big improvement and early blocks can go .100"+ overbore. Good luck.