I'm building a M-51 using the kit of Academy. I am aware that it is not the best kit of the subject, but it is already in advanced state of building so I'll convive with this.
I've fixed the most obvious defect, the enormous muzzle brake, by cut and paste of the original pieces.
Now, I think to have read that this kit represents a version of 1973 or even later. Though, the crew has helmets of US ww2 type.
Should I change the heads of the crew with the US helmet of the '60s?
Is there any other suggestion for (unexpensive) corrections and painting of the kit?

The base kit is ok for 1973, but there are some minor issues like the size & detailing of the hull-side stowage boxes. I ignored much of it in my build, concentrating on the muzzle brake and some front-end details. The figures are weird - badly disproportionate for genuine humans - but I cobbled the parts together to make one adequate figure. Israeli gear was a mixed bag, so I did not worry about the helmet.

Mine was in a Campaign a few years back (You can see it here at the bottom of the page, and completed a few pages further on.)

Hope this helps! (If you want accurate, get a DML kit...)

I think the Tamiya is a better basis than the Dragon effort, surely? Mr Hazzard based his last M51 on that and he is far more knowledgeable on the subject than I.

Possibly, but I haven't seen one myself. I was just pointing out that there are more accurate options than Academy out there...

You both are certainly right about other kits, but I am building a kit of Academy and I would know how to complete it in fair way. Throwing it and buy another one is not an option for me.
I would know more about the issues of the front and of the stowage boxes.

Fair enough. The issues at the front are mainly missing detail - if you looked at the pics I posted in that thread you'd see where I used white plastic to make up the steps and shackle retainer pins on the bottom of the nose, and the missing braces at the fenders. The odd loop of plastic is a bracket to hold a loop of barbed wire on the nose, and the "rhino horn" sticking out above it is for the wire to hang on. (Made that horn from a sprue ID tag, whittled down...) I added a tow cable from an old Italeri Sherman too.

The stowage boxes are all the same length in the kit, but the real ones had longer boxes at the front - you'd need to add about 2mm at each end of the forward boxes. And the definition of the moulded-on straps and missing padlocks could do with upgrading.

Toward the back on the left I added mounting pegs for the spare road wheels (from lengths of sprue), and on the turret I did some work to the shell-ejector hatch on the side (the hinge is missing, and the whole area is a single casting, so it needs blended in). The mantlet cover on the main gun was terrible, so I added all the securing strip details from plastic and then covered the plastic "canvas" with tissue soaked in glue to get something that actually looked like crumpled canvas. IIRC there was a big gap under the mantlet that I had to fill with plastic, but the details are a bit fuzzy.

I'm not sure if mine mixes features from "early" and "late" M51s (I'm no expert on them), but it looks the part as far as I'm concerned.

Thank you, I think to be still able to make the most part of these improvements on my kit.
About the link in your post, my browser advisor gives a strange warning that doesn't give with other pages of the site. I wonder what is the problem.

Same here...

Yeah, mine does too, but I found that old thread by searching within Armorama! It works and is safe, but clearly there's something odd about website certificates going on that my AV software finds objectionable. As I don't really understand these certificates, I just plod on...