Does your block have the valve relief notches? If so, you'll be OK with 670 heads clearing the block.
I ran 670 heads and 17 heads on a 350 with headers. I didnt notice much of a performance difference, with the XE256 camshaft. Ran within .2 at the track. Likely you wont notice much if using a moderate camshaft grind. With a 350 and moderate cam, compression is more important than head flow. They don't need as much airflow. Find a set of 72CC heads, even if they're small valve. With flattop pistons, you'll be in the low to mid 9s for compression.
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I could explain all this to the girl at the parts store, but she'd probably call the asylum.
White '67 LeMans 407/TH350/Ford 3.89... RIP
Red '67 LeMans. 407/TH400/Ford 3.25
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