The following info is from Marcus Edell (via The H.A.M.B. Messageboard)
First year was 1949 the CID was 331 OHV V8 same year the Olds 303 came out.From 1949-1962 this design engine was used (49-55 331CID) (56-58 365CID) (59-62 390CID)All intake manifolds,Camshafts,Water Pumps(diff outlet locations vary through the years),valve covers,heads,valley covers will fit on all models(Studebaker rocker arms will fit also)(valley covers will fit up til 1967)(6×2 intakes will fit until 67).The first 1949 models had valve covers like the Olds and are pretty rare now.1949-51 had some real small exh ports and small valves,all 331′s didn’t breathe too well.Also compression was low on these like 7.5:1 and only had a two bbl carb until 52. 6 volt until 53.
1949-1954 Cads were offered in manual also.And used the same trannys from 37-54 including La Salle,Olds,Buick..they all will bolt up the tailshafts are different etc.Oldsmobile shared the same set up until 1956.The 331′s could be bored out alot up to a 1/4″but 1/8″or 3/16″is more practical.
Basically a 390 is a bored out 331 and 390′s can’t be bored out that far.All Cads use non-adjustable rockers which limits you on any type of performance cam,adj rockers are a must in a performance motor and still can be had,just let me know(stock valve covers will not clear,either shim them up or use Edelborck or Hildebrandt covers,Moons and Offys wont clear as well).All Cads 1949-1958 use a partial flow oil filtration system,the 1959 390 were the first to go full flow something to consider when choosing a Cad to build.Distributors are the same 1949-1954.All in all the 390 is the best for performance choice of them all,they look the same as the early models and will give you the cubes and compression tons of torque and horsepower!
Cadillac cool stuff-Early valve covers have the cursive script and were used I believe until 1955.Edelbrock and Hildebrandts are what you need for perfromance and clearnce.
Distributors-everyone made them but are really hard to find now Du Coil,Grant,Spaulding,Rotofaze are the shit but good luck getting parts.Mallory Magspark was a great unit and the parts are still being made ie points,cap and rotor!Also Mallory will build you a new custom unit as will Vertex or Joe hunt.
Pistons-I know Jahns and JE will build you pistons of any bore and compression,Jahns will only do cast though,but for the later mill ie 390′s,stockers from TRW,Egge etc will work fine and give you 10.25:1,more since you will surface the heads.
Cams-Back in the 50′s and 60′s Isky made some cool rollers,and I believe there is someone making them again.My cam grinder wouldnt tell me who though! Cads dont like much more than .475″lift and 280 adv dur remember adj rockers will be a must at this lift.Can use Chevy lifters but need to have pushrods made,also can be done for like $8.00 a rod.The aftermarket Cad lifters are junk and made overseas.
Heads- 59-62 390 headsare the best and will bolt onto the earlier models no problem.Must use the rocker shafts from the 390.so make your mind up first about using adj rockers before you order them because the 331′s are different than the 390′s.as well as headers at this point eraly cads are small and rectangle the 390′s are large and sq.the 390 can be fitted with Chevy 2.02 or 1.94 intake valves and 1.60 exh.to balance the flow properly leave the stock exh valves in the center two,because they are siamesed and share one large port.
Intakes-Cadillac made a cast iron 2×4 used 55-57 models as well as a 3×2 set up used on the 58-60.the 3×2 works ok but none of them really give you the performance you can get from a good aftermarket unit.The intake that flowed the best was the Weiand model 2×4′s in the 60′s that featured the large AFB carb pads the earlier versions were made for the WCFB small base and didnt flow worth a shit.The first aftermarket intakes were made by Detroit Racing,they featured a dual intake that really worked well as did Edmunds and Eddie Meyers,Offy,later a 2×4 was made this is more for a 331.3×2 intakes by Edelbrock or Weiand are the shit,sorry Offy! 3×2′s seem to work well on Cads also.4×2′s Horne made the first they later became Cragar,pretty cool set up,but Edelbrock shook things up with a staggered design that delivered!6×2′s by Weiand and Edelbrock again rule especially the Dragstar by Weiand and the Edelbrock CA-6 they are high rise look bitchin,thats for all out balls to the walls throttle,one purpose only Fuel and more fuel.The Offy is ok but they are low rise only,Cragar made a cool low rise to.The 6×2 will fit the 63-67 390/429′s as well,the deck width on these engines are narrower just cut the 5/8″fuel hose to fit and voila!2×4′s by Weiand in the 60′s were the absolute best all round intake,Nicson,Detroit Racing and Maybe Edmunds made a 2×4 as well.Blower intakes were made by Cragar I dont know if the rest made one or not,alot were just fabbed up
Fuel inj by Mr Algon and Mr Hilborn were made good luck on that search.