hey guys...i have another question, im wondering what kind of modifications will have to be done to the 350 heads besides drilling the 7/16 bolt holes out to 1/2" to fit on my 403??

thanks for the help!

i beilve that would be it...

You may want to check out www.oldspower.com there are some persons with that same combo, they could probably give very detailed descriptions there.

Enlarging the head bolt holes will allow you to bolt the heads to the 403, but you may want to install larger valves as well. The #7s came with 1.880" intakes whereas the 403s had 2.000" intakes (which are the low-perf big block valves). While you're at it, you may want to install the larger 1.625" big block exhaust valves as well. The problem is that by cutting the #7s for the larger valves, you'll loose the factory induction-hardened valve seats.

Also, I assume you know that the 403 had big block sized 83cc chambers originally vs. the 68cc chambers on the #7s (which I assume is the reason you're doing the swap).

ya i knew the #7 had smaller cc's then the 403 heads...that is why i am making the swap...

thanks for the help!!

A friend of mine did this last summer, the head bolt holes needed to be drilled, he had larger valves installed, and he needed longer pushrods to make the valvetrain geometry right. Nice bump in compression.

You shouldn't need longer pushrods, since the valve tip installed height should be identical. Unfortunately, most machine shops don't know how to set up Olds heads correctly and chances are the valve tips were set wrong when the heads were machined for the larger valves. The Olds specialty houses sell a tool that allows you to check the valve installed height using the same process as the factory service manual.