I have a 1951 Plymouth Cranbrook and Im running a stock
Flathead straight six. I want to swap it with a small block but
I'm undecided if I should stick with a mopar small block or go with
something else. Any suggestions???
With all the goodies on the market today, you can put just about anything in it you want. If you want to be a purest, stick with a mopar engine, just to drive the Bow Tie crowd nuts. Seriously, a good engine to drop in would be a mildly warmed up mopar 360, with an automatic. Since they arent the most popular engine around for swaps, you can get them fairly reasonably priced, and there are a lot of goodies out there for them at competitive prices with the Chevy and Ford stuff.
The best thing to do with an old Mopar that you're not going to keep original is to put a stroked 400 in it. At factory bore you end up with about 450 CID. Talk to your machinist, he'll tell you how simple it is to do. If you don't want to do that, then the 360 is probably the right choice.
If you really want to do it differently, go with a slant six and find some of the old factory hot parts for it. In the early 60's, mopar made factory headers and 4 bbl manifolds for the slant six, as well as a couple of optional cams. That would make a true retro rod as well. While the slant sixes came with some rather restrictive valves, even a little porting and a good 3 angle valve job will really wake those heads up. The 225 slant six with 4 bbl from the factory was rated at 196 hp @ 5200 rpm and 212 lb ft torque @ 4200 rpm, so it was some what of a high winder. And this was with 8.2:1 compression.
That is compareable with a lot of the small V-8's at the time that were 40-50 cubes larger.
You might also want to check with Clifford Performance, they are 6 pack specialists.
An afterthought, the last couple years the slant six was in production, they had opened it up to about 240-250 cubes if memory serves me correctly. That was in the late 70's early 80's.
While the 400 etc would definately be a tire roaster, I fear that your early drive train might not handle the strain of that much torque. At any rate, I would recommend going with an automatic, as it will be decidedly easier on your rear gears. etc.
You've been given some great suggestions so far. My opinion would be to stick with Mopar whatever you do. Anyone can own and drive a cookie cutter small block Chevy powered rod and most people get really tired of seeing them. It's kind of like the SBC is the brainless way to go.
If you're building a cruiser then my suggestion would be a 273-318 Mopar. Lots of speed equipment available and respectable performance. If you want something hotter then you can opt for the 340-360 versions. If you want a pavement pounder then a big block is the way to go. Mopar big blocks, especially the 383-400 are a dime a dozen and with very little work can roast the rubber off almost any tire combination. Finally if you want class, then an early hemi would be the ticket. For the Cranbrook I would suggest one of the Dodge or DeSoto versions since physically they are a bit smaller than their Chrysler counter parts. A hemi would be a bit more expensive to build but real class never comes cheap.
Whatever you do, please don't do the "brainless" swap and keep your Mopar all Mopar.
I also agree with Centerline about staying Mopar. Anyone can be a clone and wear a Bow Tie. Dare to be different. I dont know how many old Red Ram and Fire Dome engines are out there yet. These were the hemi predicessers to the Venerable FirePower Hemi of the good old days, which even today is copied for all out Drag engines. The Red Ram and Fire Dome were around 245- 361 cube engines, and not a lot of stuff available for them these days. My recommendation would be, what ever you decide to do, go with a later engine, and save yourself a lot of cash, in chasing goodies.