The manifolds in your photo that do not have stiffener struts between the ports are notorious for warping after a lot of heat/cool cycles and putting tension on the attaching bolts. if they're ever unbolted from the heads they spring inward, and can't be reinstalled without a LOT of prying and possibly cracking. Later versions of the same manifold had a 1/2" square strut cast between the ports to prevent the warp. BTW, dyno tests I've run using that style of manifold have outperformed a bunch of tube-type headers, especially in the low and mid-RPM ranges.
The 4.3L Chevy V6 used in the S-10 trucks with the 700R4 transmission are plentiful and cheap, if you're determined to replace the 327 with something else. That would be sort of like throwing out a good Sirloin steak and eating a hamburger, though!
Jerry
