I highly doubt you will fit any factory style ROUND port manifold in 65-66 B body, there will be great interference with the power steering gearbox, and possibly the frame, motor mounts, etc. Now if you had D-port heads, you could use long branches easily.
Your best bet with round ports in a 65-66 fullsize is to give Mark at Mad Dog Headers a call. I recently put his part # 9169 (modded to 3.5" collectors) in my 65...fit is great. Does require a small starter though.
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Red\black 65 Grand Prix 467 E-head stroker, Viper T-56 6-speed, 4.10 spool
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