1....You can NOT bolt a multiport magnum intake to any LA engine, and the manifold CAN NOT be redrilled.
(If you spend enough time AND MONEY, you might be able to re-drill the LA heads for the magnum intake, but that would require removing the heads)
2....Magnum head swaps have been done, don't know if practical on 273, due to compression.......would mag head drop comression? Don't know
Go over to "Binder Planet" and read some of the ideas over there. Guys HAVE put injector bungs into carb manifolds and ran multiport injection with older manifolds.
Magnum "stuff". You need a mag crank sensor setup which you will have to build. You are not going to be able to run the factory magnum flexplate on your 273, at least not without a lot of work
In theory, you could run a mag distributor which provides the cam sync (not ignition trigger)
speed sensor.........Not sure..........there were some TBI engines which used speed sensors which fit the early style transmissions, so "maybe" you could go with that. SOMEWHERE I found a NP435 truck gearbox out of a TBI truck which had the "same as" later speed sensor. AKA the speed sensor fit the 4 speed and worked with the factory TBI and could be used with newer multiport
You could build up your own TBI.......get / find/ steal/ buy a TBI throttle body, buy a megasquirt unit. You don't need cam sync or speed sensor, and could trigger it from a Mopar electronic distributor.
Find/ look/ wait a USED unit. I have an old Holley pro-jection, a Holley Commander 950 (I would sell electronics to that) Holley HP adapted to the Commander TBI, and have just bought an FItech TBI "used" off CL for about 1/2 the new price Yeah.........it's a gamble
This all started in 2012
Anyone know Holley Commander 950's? programming?
This is the old Holley pro-jection. Computer works but not worth the trouble. GREAT candidate to upgrade to Megasquirt. Payed 75 bucks I think
Used Holly Commander 950 I think I paid 300 bucks. Turned out to be a steal, you cannot find the TB unit for that on egag Only thing really wrong with these is they are not "self learning"
Later, I upgraded the Commander to Holley HP for around 1300 "all new" from Holley, with HP computer, harness and O2 sensor. EITHER the Holley HP or the Commander computer (or Megasquirt) will operate EITHER TBI or multiport.