Remember that the Poly A motor has the basic block of a "double-rocker shaft" V-8 under it. The 1952 Chrysler "Hemi" V-8. Which sat in a chassis architecture designed for inline 6 and 8 cyl engines. THIS fact explains the restrictive exh manifolds on the early Hemis, which can't be seen from the top, but are painfully apparent when the engine is on an engine stand. The A-body Valiants, Lancers, and later compact Darts did not exist until 1960. It was 1967 before Plymouth cataloged the B-block 383 in Barracudas and 1968 for OEM-production Dodge Darts. Those early A-body B/RB motored cars had highly-compromised lh exh manifolds to clear things like steering and such. Even to the point of only manual steering.
In any Unit-Body construction vehicle, the engine is installed from the bottom, not from the top (as usually happens in repair shops), so total engine width is an issue. The LA engine shaved a few pounds here and there, plus the normal cy head configuration helped trim the width and weight. Still, a kind of flaky lh exh manifold.
Everybody remembers the "Mr. Norm's 383 Darts in 1967", as the OEM-production 383 'Cudas slid in under most radars. So, the A-body cars had engine compartments designed to take the width of a B-engine from the start, but only Slant 6s lived there.
In the early 1960s, like about 1962, drag racers had started to put RB drag race motors in Valiant station wagons, without much effort at all. They wee "tight", but in there. Pictures in HOT ROD Magazine, too. Probably easier to pul the engine to put spark plugs in it, but a similar situation existed in the Chevy Monza V-6s of the middle 1970s.
Even the 426 Hemi will fit in an A-block. It is very easy to see which A-body with a 426HEMI in it, that started life as a B/RB engined A-body. If it started as an LA-block car, if the mounts are not massaged, the rh exh manifold will be like 1/2" fron the rh fender apron.
One of our North Loop Dodge Perf Team members had a '67 'Cuda 383 car he bought new. So his stroker HEMI fit in nicely, but one I saw at Mopar Nats a few years later, had the "more-offset to the right" installation. VERY visibile, even if you don't know what you are seeing.
So, two things "killed" the Poly motors. One was amount of cast iron needed to cast the cyl head and the added production cost of its "different" rocker arm arrangement. Less cast iron material needed in the narrower LA heads.
Side issues included the need for "more displacement" in the marketplace. Everybody was moving in this direction and the Poly was at a good spot, but not in a spot that could be expanded upon. Enter the B/RB engines for the 1958 model year. With the "B" being 350cid and the RB being 413cid, which included many "class-leading" design orientations. With "class-leading" meaning the new-for-1955 Chevy V-8. As it turned out, though, Chrysler "upgraded" the design in several respects (distributor at the front, shaft-mounted rocker arm assy).
Production costs are always a sticking point, but Chrysler's financial health resulted in a big shake-up of upper management in late 1960, which is why DeSoto was deleted mid-uyear in 1961. Plus some other combinations. So work probably began soon afterward to "cut costs" on the A-engine platform and standardize it across several carlines.
Enjoy!
CBODY67
