Tubular k vs factory k

Professor Fate said

If the headers fit before with a 273, and they don't fit now with a 360; then you changed something else or you have something installed incorrectly. There should be no issues now with header fitment that weren't present before. Did you re-use the 273 mounts, or buy adapter mounts? Which ones? New or different biscuits? Which ones?
More info and pics or at least a description of the trouble area(s) would go a long way in figuring this out.

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Okay, so you've changed a ton of stuff, so comparisons to previous fitment is irrelevant.
You still didn't mention what engine mounts you used, the one picture that showed the DS mount looks like an aftermarket bracket- are you sure you ordered or recieved the correct ones?
You can also try jacking up/hoisting the engine another half-3/4 of an inch off of the pedestals to see if that improves your clearances. If so, either shim your mounts or install the thicker 1980ish truck mounts (about $5, a heckuva lot cheaper than a tubular K frame).

And make sure your mounts are situated all the way to the passenger side, the pedestals are slotted and allow you some degree of adjustability.