This is a good point and an interesting way to look at it. I don't have any good data, so I will defer to rumblefish. I see what you're saying though.
As a teenager, I did put 360 heads + 4bbl. on a 318 and I "thought" I was going faster. I never did it the other way around + I've "heard" that the newer "magnum" 318 heads are better than the old LA motor 318 heads....that's all I've got.
I do not have hard data like IQ52 does. I did the 318-360 head swap decades ago. The seat of the pants said more power.
As far as the mismatched ports are concerned, depending on the builds power level, I wouldn’t worry about it. But there is significant power to be found by ether porting the intake to match the heads or the head to the intake. But again, this amount is a variable as build combinations are almost endless.
Some will say it is worth it and others will disagree. Part of the issue is cost involved in the work to match ports Vs. the power return.
Speaking strictly on a 318, I wouldn’t use a 360/340 head on it unless it was reasonably cammed and had some decent gears to back it up. A stock or very mild cammed engine (IMO) will do just fine with 318 related parts.
Speaking strictly on a 340/360, I wouldn’t use 318 heads on it ever unless it was all I had and have zero money.
A larger intake (340 ports) on a small port head (318) is OK for general useage. I wouldn’t bother trying to make a Performer out of it. Being the heads port window gets in the way of the incoming air and fuel charge, it does crash into the head. It’s a wall. And the fuel just starts to gather, then puddle in. This is crappy for low speed driving. An engine likes a fine well mixed up charge at idle & low speeds.
To nut shell it, if your going to build something for any type of performance, do it right and don’t skimp. You will be a world ahead and way happier.
On a B body, I would only enhance a 318 for minor power and mileage concerns as the B body is a bit large and weighty. If it was in a lightened A body, you could have some fun.
