I have heard from others who did this swap that even though the mopar/amc lifter are listed under the same part number, to insure success you should get the amc lifters because there IS a difference. I have also heard that there have been some problems with correct pushrod length especially if any material has been removed from the block or head and that you should assemble your package and carefully measure for length. Several guys have found incorrect preload on the lifters which resulted in noisy or clattering lifters.
Magnum valves and valve springs and retainers are different than LA and are also more expensive. Unless you use Hughes retainers, you will need to cut the valve guide bosses if you plan on running a cam with more than .525 lift as there wont be enough clearance. I have also been told that magnum stock valve springs are only good for about 5300 rpm. I would also check overall chamber volume with the swap to insure that you aren't actually lowering your compression ratio with the swap.
On a positive note, you will gain approx. seven percent lift by using the using the magnum heads as they have 1.6 rockers as compared to 1.5 LA. So that would effectively bring your 480 lift to about .512 which is really close to the limit without machining. If your block or heads have been cut much this could be a problem unless as I previously mentioned you cut the guides/get retainers. I have a .512 lift in my 360 mag which only has about 30 thousandths clearance. Between the lifters, springs, pushrods, pushrod measuring tool, gaskets,magnum head bolts, magnum intake(unless you redrill yours) etc etc.. you could wind up shelling out some major cash.
I don't know the flow figures on a 77 smogger 360 head that have been ported but have heard from others that the magnum heads respond very favorably to porting by using the MP templates. If you are prepared to do all your machine work as you say and also port the heads in combo with the increased lift this could be a good deal for you. But I would also look at how you could get a bigger bang for the same money and less hassle for example stiffer gears.
