If you only want 350HP and you really want to keep the swap simple go with the 91 360 it has good heads on it already they should be 308 castings a lot of super stock racers used those in the past yes highly modified just like the other heads in super stock but there is a reason they used the 308 castings over the older castings they made more HP.
350HP from that 91 should be pretty easy to do with a camshaft regrind a RPM intake and headers.
Im not sure but seems like the accessory mounting bolt holes are a little different on the Magnum and LA heads so im not sure if all your LA brackets for the alternator and P/S and A/C are going to work with Magnum heads without modifications. I have a set of Indy/RHS X heads on a LA engine and they have the Magnum accessory holes on the ends of the heads and I had to modify my LA brackets to work with them not a huge deal but I had to modify one bracket cut it and make it shorter and weld back together and make another bracket from scratch.
Also not sure how well LA headers that you will be running in your Scamp match up with the magnum exhaust port most say they do though.
For 350HP the LA is the easy swap. I don't see the Magnum being that much better than a 91 360 myself yes it has a half point more compression (run thin head gaskets to make up the difference) either engine will need the cam reground or changed, both need a RPM intake and headers. Heads are not that much better on a Magnum compared to a 308 head casting 360. Main advantage a Magnum has is 10 bolt valve covers get a good set of cast Mopar Performance covers with a set of Super Performance gaskets.
Yes the Magnum has a few advantages stock vs stock but when you start hot rodding and he has 308 casting heads already on the 91 I would use the 91 the Magnum heads all seem to crack also even though it doesn't seem to cause any problems in most cases.