As far as the engine goes, I wouldn’t worry about the heads, the early HEMIs came with pressed in, replaceable exhaust seats. I’m told they might not be as hard as the current replacement seats, but they are definitely hard enough to prevent problems associated with running unleaded fuel.

At 73K on the motor and not smoking I would look at the following.

First would be oil pressure (when hot), if that’s good you probably can get by with the motor pretty much as is but there are a couple more things I would recommend. One is to look for sludge build up (in the 50-early 60s non detergent oil combined with a road draft tube ventilation could lead to an engine that has a lot of crud build up) and address that problem if it exists. As designed the HEMIs also used a By Pass type filter system. I would recommend the By Pass valve be removed and the replaced with the plug Hot Hemi Heads sells (requires rear main cap removal) and possibly also change over to a spin on filter at the same time. If you decide to change oil pumps while you’re in there I would recommend avoiding the HV Hot Hemi heads unit which I have been hearing has had some major problems with the shafts breaking (they basically use a HV 340 oil pump with a custom shaft). If you can find a stock 392 oil pump it will flow within 3% of the HV 340 unit, but may require a custom pickup depending on the pan you’re using. On a final note on the Oil system, most of the old timers recommend using a fairly heavy oil (20-50) due to the clearances of these older engines. If you go with the heavier oil, plan on giving it a little warm up time when starting the engine cold.

As far as power, the big thing that really hurts the early HEMI is the lack of compression, but comparing the power output of even a stock 354 to “a good running 283” is a little on the light side.......it’s a bit closer to a good running low compression 350, but with a weight penalty. Couple that with a set of 4.10s and a OD 4 speed and even in stock form it will still be a fun ride.

Here are the links to the HEMI threads I did several years ago you might find of use.

General Hemi Info Part 1

General Hemi Info Part 2
General Hemi Info part 3

General Hemi info part 4

General Hemi Info Part 5