newbie here. asking for anyone who has replaced their 70's vintage olds 350 with a 403 short block. trying to plan a project! thanks!

 

It’s an easy swap, should be pretty straightforward.

 

They are externally identical. The only substantive difference is that the 403 uses 1/2" head bolts, so you will need to open up the 7/16" holes in early heads if you are going that way. Of course, the 403 is the lightweight block with thinner walls and windowed main webs, so not as strong as the 350 block. The cylinder walls are thinner, so overbores are limited.