Poly Engine what was good/bad about it and why is the LA better?

Some good discussion about the A318 here. There's a lot of confusion and misinformation about them online and by word of mouth.

One fallacy is that they are drastically heavier than the LA. They really aren't. Someone on here a while ago actually weighed an LA318 block and heads and an A318 block and heads and determined the A318 is only 30 lbs heavier. The earlier 1957 - 1961 A318 would be a little heavier since they used iron timing covers, so maybe 15 lbs more? Weiand made aluminum 1x4 and 2x4 intakes for the A318, so the cast iron and aluminum intakes are comparable in weight to the LA options.

Another misconception and nickname the A318 got was the "wide-block," and some people even call them "big block 318s," which makes no sense to me since the blocks are nearly the same dimensions as an LA318. It's the A318 heads that add about 1" to the width. 30 lbs has branded them "heavy" and 1" has branded them "wide." If weight savings was the excuse to run an LA318 versus an A318, it would have been easier for the driver to go on a diet :D If 1" width made the difference, I'm not sure how the factory fit B-blocks in A-bodies.

There are two types of A318. Plymouth came out with the A318 in 1957 through 1958. As someone already pointed out, they offered a high-performance A318 called the V800 that had dual quads, hotter cam, and a rather accurate 290 HP. In 1959, Mopar offered the A318 in all divisions. In 1962, Mopar redesigned the A318, which stayed the same until they stopped production. The 1957 - 1961 A318 shares the same bellhousing pattern as the early hemis and don't have a lot of parts that interchange with LAs. 1962 - 1966 A318s share many parts with LAs including bellhousing bolt pattern, crank (except 360), balancer, rods, bearings, oil/fuel pump, oil pan (except 360), distributor, valve springs and hardware, timing set, rings. The timing covers can be made to interchange, although the timing marks are on opposite sides. Heads, intake, exhaust manifolds, and pistons won't interchange. There are tall tales of people supposedly retrofitting A318 heads onto LA360s back before 4" stroke cranks were economically available for the LA273/318/340, but I've never actually seen evidence and think it's likely hogwash. There are both oiling and cooling issues that would need addressing. Now it's far easier to simply stroke to 390.

As some have already mentioned, Gary Pavlovich is one of the prominent A-block builders and enthusiasts still around. He wrote some informative articles back in around 2000 or so that got me to reach out to him; he's been one of the most honest, genuinely helpful hot rodders I've ever met. I spoke with him a few months ago, and he finished his own 390 stroker making 600 HP twisting 7,000 rpm. Obviously quite the build, but it's pretty normal now in the right circles to see people build 450 HP A-block 390s. With those valve covers, I'd take that engine any day over a 450 HP LA.