Magnum heads oil differently than LA based heads, and the magnum water pump turns backwards(?).

How much better are the swirl port heads? If the difference is minimal I'd just buy, or modify, an LA style carbureted intake for the magnum.

Magnum heads seem to be pretty prone to crack between the valves. So long as the valve seat isn't affected and the head isn't leaking it's no big deal. There are a ton of vehicles being driven around with cracked magnum heads. If your not going to use them you can sell them to help recover some of your money.

90k is worthy of a new timing chain + a new oil pump pickup tube. Maybe even do a new oil pump while it's apart. I'd knock out the steel freeze plugs and go with brass instead. The rest of the engine should be ok.