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LP gas (Propane/Butane mixture) is approximately 110 Octane. That's a good thing- - - -you can bump the compression way up (12:1 or so) without detonating IF you can find the right pistons. Don't even think about milling the heads- - - -they're virtually flat, and the "combustion chamber" is actually in the top of the cylinder with that oddball angle between the top of the block and the cylinder.

Downside- - - -LP displaces a lot of volume of air as it mixes with the air when it goes through the throttle body. That reduces the horsepower you'll get on gasoline by about 10% or more. LP is also rough on exhaust valve seats. You need Stellite hard seats on the exhaust valves if you ever intend to use the truck like a truck. If you're just tooling around in "Cowboy Cadillac" mode, you probably don't need hardened valve seats.
Jerry

Jerry why is it hard on valve seats? This truck has allegedly been running on propane since the motor swap in the late 60's early 70's and it was used hard as a tow truck till like 4 years ago. That's 40+ years of service, should I pull the heads and have a look at em? Like I said it's running like a champ now with plenty of power.

Spartan

Chances are, whomever installed the engine had the heads reworked when they did. LP is a "dry" fuel- - - -there's none of the lead anti-knock compounds in it that the gasoline used back then had, and the byproducts of burning tetra-ethyl lead created deposits in the heads that helped prevent valve seat erosion. LP engines have always had to run Stellite seats, at least during the 60-something years I've been building engines.
Jerry

If the engine was original on propane, might have the higher compression pistons available. Still, less HP than with gasoline. Propane engines last longer because no 'washing' with gasoline. I would say keep it propane.

Ed

What part of what I said was wrong? Why didn't you notice that the math is off?
Have I ever heard of a billet crank? Remind me exactly where this reference was made? What I read was "stock bore 409 with a 454 crank". I took that to mean "a crank for or from a 454". A billet crank has no dimensions at all.

Panic is right on the math. Just got off the phone with my 409 nut friend and he straightened me up. The 409 with a 427 crank (? bore) is a 447 and the 454 crank plus 0.060 bore gives 487. Turns out the 454 was the cheaper route because of the counter weights size. The 427 took way more Mallory (heavy) metal which is $$$. If one wants more detail PM me and I'll put you in direct touch with my buddy but be prepared for a l-o-n-g conversation when the topic is 409's. The head details alone are good for at least an hour. Be very comfortable if you ask about Roots type blowers and electronic fuel injection. He's done it all.

It's also possible to make a "426" MOPAR hemi a "547"- - - - -I've built a couple of them. When I was finished boring the epoxy-filled block for a set of flanged sleeves, I had stripes of epoxy showing where I'd bored all the way through the cylinder walls in places. I don't remember what stroke we used, but it wasn't stock. We put coolant in the cylinder heads only, and moved the truck out on the pull track with a Jeep and a tow bar before hooking to the sled. Then the engine got started.
Jerry

Single most common mistake w/r/t displacement: remembering part of what you did, but not the piston oversize.
409 stroke: 3.50"
427 Z11 stroke: 3.65"
427 Mk II stroke: 3.76"
454 stroke: 4.00"
348 bore: 4.125"
409 bore: 4.3125"
427/454 bore: 4.251"

409 bore + 427 Mk II crank = 439"
409 bore + .045" + 427 Mk II crank = 447"
409 bore + .060" + 454 crank = 481"