The Laker photo above showing prominently the centre landing gear of the DC-10-30 displays the reason why the Tristar was a dead end for long range ops. Both were designed originally against a US transcontinental requirement, without much expectation of intercontinental use. The initial DC-10 structural design envisaged that one day this extra middle gear might be needed for greatly increased weights, and provided for it, whereas the Tristar didn't, and the only way to handle more fuel load was to shorten the fuselage. Incidentally, the middle gear could be readily removed, Japan Air Lines in particular did this as they rotated their DC-10s (and MTOWs) between domestic and longer-haul fleets. The DC-10 middle engine position also made handling a larger later engine more straightforward, whereas the embedded Tristar one did not.
Douglas were the past masters at stretching, generally encouraged by the airlines, and their initial designs envisaged it. Lockheed hadn't done a commercial type for more than 10 years, hadn't sold to overseas airline customers for longer, and designed down to the original spec.
The best US aircraft ? Technical design by Lockheed, manufacture by Boeing, sales & marketing by Douglas.