$125 is a fair price - unless you have already had the heads fluxed and a valve job. there are a lot of concerns about aluminum heads; they could be tweaked (warped), requiring milling, electrolisis could have eaten out the water ports, the seats could be leaking.
I'd ask; why were the heads removed??? what's wrong with the rest of the 300? - how many miles?
I just did the heads (valves) on my 300, rebuilding the carb and putting it back together. then, if i decide to sell it (to drop the 455 in the LeSabre), I have a running 300/400t to sell as a package... and it won't be cheap...