I am considering buying an old firetruck to convert into a truck to haul grain. IT has a 534 in it. I have heard that engine was made more for running in place for long periods rather than going up and down the road and is not good on gas mileage, is this true? I have also heard that a 429 can be put in its place without much trouble, is this true?

 

Well, this is my opinion and my opinion only, but........
The 534 was a good motor......in it's day. But that day has come and gone. I think the 429/460 is the best logical gas engine that you can replace it with. The 534 was made when large trucks would go down the road at about 35 mph and not much more. It's a gas hog (I guess the 429/460 is also) weighs about twice as much as the 460 and has about half the RPM potential. The 534 has no speed parts available for it and replacement parts are really only found in the salvage yard or maybe on ebay.
Consider replacing both the engine and tranny combination and of course your motor and tranny mounts will probably have to be engineered by you.
While you are on the subject, did you ever think about putting a diesel in it instead of a gas? Just a thought.