273, or 318?
I am with Dan on this, a numbers match car is worth more. The 273 engines run well, so I would be inclined to rebuild it. Either get David Vizard's How to Port and Flow Test Cylinder Heads, and his Build HP books, or watch his Youtube videos; David Vizard Powertec 10. Most is Chev or Ford discussion, but the principles apply. He is involved with Andy Wood of Unity Motorsports Garage and Uncle Tony of UTG, on a project called Mission Impossible. Another porter, Charlie Servido, might bbe misspelled is doing head and port work. A number of Mopar oriented videos to pick up little bits of wisdom.
For valves DV is starting with the stock LA 318 valves and is then going to test a set of used Chev LS valves to get larger valves with lighter weight. LA engines have 3/8" valve stems while the LS use 8mm stems. The Mopar Magnum also have 8mm stems. It would be interesting to compare valve weights between the LA, Magnum and LS along with installed heights.
With regard, for an example, the SBF and Chev use 11/32" valve stems. Before the LS Ford guys were installing Chev 1.94/1.5 or 1.6 valves. The Chev valves are a bit shorter but the keeper groove is at about the right height. The LS valves are about the same length and the keeper groove is at about the same height. A 1.9" LS Intake valve is about 20g lighter than the 1.78" Ford Intake valve. The 221 and 260 Fords used 5/16" valve stems but small valve heads. These two engines suffer the same situation as the 273, short number of production years, so parts are harder to find at reasonable costs. I would consider putting 5/16" bronze guides in the 273 heads which can then be honed the 0.0015" to 0.002" to fit the LS or Magnum valves.
Back to the Ford heads, 63 to 67 289 HiPo heads and 68 and on SBF had 1.78" spring installed height. LS valve springs with their 1.8" installed height work. The 2V 289 had an installed height of 1.64" or 1.66" which posed a bit of a problem in spring choice. A call to PAC Racing Springs solved the part number problem and ordered through Summit. Jegs or another supplier can likely get them.
For your one bad cylinder, you can have the machine shop sleeve it and finish the bore to match the other seven.
Just some ideas to think about. Compare the difference in cost and fit between the Magnum and LS valves along with the springs, retainers and keepers required. Also consider that the out of pocket today will be spread over 20 years of happy, snappy and reliable driving.
Since it's a all original number matching car, I would go with the 273 as originality is worth more money.