340 in 66 cuda?

A 340 is an easy fit. Use regular 66 V-8 motor mounts. You will need to weld a little triangular piece on the right V-8 motor mount and drill it so it looks the same as the other side of the same mount. The clutch bellcrank may ned to be shortened to fit against the larger V-8 bellhousing if you have a 4 spd. The exhaust manifolds or headers are the biggest issue. Another issue is finding a really sanitary route for large dual exhaust pipes you will probably want. (Not a big deal but worthy of a little planning prior to the swap) If you use a 64-66 273 water pump it looks and fits exactly like the original 273. You might need a bigger radiator if you jump horsepower on your 340. Mine kept cool when the 340 was stock but when I went to 400HP it wouldn't keep cool in stop and go traffic. I had to go with a better radiator. The 7-1/4 rear end will probably last until you engage the four barrel on your 340. Plan on upgrading to a 8-3/4. (I've got a pile of about seven 7-1/4s with the pinion cross shafts torn out of the housing if your interested....Slow learner here......really embarassing to have a 5.0 Mustang beat you as you clunk to a stop at the side of the road.... ) A little rewiring to adapt to electronic distributor and voltage regulator will also be highly recommended. Another helpful upgrade is to go with the double wire alternator to provide the volts to ensure good fire for the electronic ignition.
A 340 in a 64-66 A-body is aboout as easy a swap as there is. Do It!