I cleaned up my 66 manifolds a bit before installing. Who knows if it made a difference. I just thought it couldn't hurt. If there was a performance increase I would guess may be a couple horsepower. The 273 manifolds, 2 and 4 barrel for any given year., are the same part numbers. Without getting into specifics, there are different casting numbers for some manifolds between 64 and 69 273's but the differences are because of design changes and not for performance. The 340 manifolds of course have bigger ports and outlets and in 72the manifolds changed again with slightly smaller ports, tubes, and outlets. From what I see in the parts books, 273/318 manifolds weren't used on 318's.
To the 66 and newer heads, yes.
To answer your original question....yes. I've gasket matched my 273/4 engine and cleaned up slag and other casting flaws so that the gases flow a bit freer. I slicked up any rough casting where I could reach with a long shanked grinding stone. Beyond that, I would have to extrude hone them for the ultimate in flow. I couldn't justify that cost for street use. I also did other engine mods, so, trying to compare a performance increase for just the exhaust changes would be too hard to determine.
Now extrude hone. That's a cool process. For squeezing every lst oz. of power out of those factory manifolds. The small block drivers manifold restriction around the steering shaft is like they say "like pooping through a straw" No way around it except for electric steering and may be a B or C body manifold.
F yeah, that works! I did the exact same thing 25 years ago. 318 short block with everything else 273-4. +45 cubes...almost 20%! Huge difference on the street. And all the A body manifolds look like that to clear the steering box, From 64 to whenever they widened the steering box mounts. 2bbl-4bbl 273-318 were all the same.
I found a 68 318 short block pulled by a local wrecker for $100 (that place was cool, different short blocks lined up, heads were on another shelf) cylinders were in great shape, no taper and still had a cross hatch. even though I lost 'compression', I picked up 45 cubes (and 80 ft lbs!) and I think that really woke the bottom end up. Top end not sure but you use the bottom end EVERY TIME on the street, top end not so much. Those were my findings. I was so pleased with it, I made it my signature......If you want to refine that package even more, get a camshaft made for the 318's lower compression. Not sure what that will get you though. It ran great as it was. Later I installed a Performer intake and 5208 headers, ran even better. Performer had to be modded for the 65 intake bolt angles: hardened washers and clearancing the holes. Worked great, no leaks.
The 340 (68' - 71') output to tail pipe is larger, flat and no cone, the sweeps are similar. A guess is the openings at the head may also be larger on the early 340's. If I were to use the best cast iron exhaust instead of headers, I would buy the after market ones for the early 340, around $600.00. Don't forget the special bolts and cone washers to go with them.
The problem with using the '68-up 340 exhaust manifolds on a '66 and older A-body 273/318 is that they don't fit the body very well, you need to custom make the whole exhaust system, and you'll have a hard time keeping head to manifold gaskets from blowing out due to the very thin sealing area on the mismatched port sizes between what a 340 head has for a port, and what the 273/318 head has.
I thought we were talking about 67' 273 heads. I have hanging onto a 65' 273 4 Bbl intake being it's rare and compatible for the early 273's. I tossed the 65' heads out last year due to its small ports. I can't keep everything.
My mistake, you're correct that the OP was talking about a '67 model. The 340 manifolds will fit that car fine, but, the port mismatch I mentioned still applies.
I agree with most of the suggestions here, the important thing is to pay close attention to the advice being given. For example, if you’re going to use a 318 short block make sure it’s a 68 or 69, so you’re getting the high compression block. Once you bolt your 920 heads on it the C/R difference is minimal, and will make it better suited to modern pump gas. If you want to keep your ride height low, use your 273 manifolds or go with Dougs or TTI headers. Since you have the standard intake bolt setup, an Eddie performer would be good, an air-gap would be better (don’t forget aluminum intakes and headers will also shed weight compared to iron anything) The aluminum vs steel wheels advice is solid... I make sure to determine the weight of the aluminum wheels before buying. If you decide to home-port/gasket match anything you might as well port everything (intake to head, head ports, and exhaust manifolds. Spec your carb carefully and once you decide on your parts list check with the guys here about a cam. I believe you could go for a Comp XE268H with what you have now plus a hi-comp 318 block. 3:23’s are alright street gears, but with the build you’re doing find a 2500-2800 TC - you’ll be surprised how it wakes up a mostly stock setup.
