Yeap a clogged heater core sounds like the problem to me. A different temp t-stat will have no effect on the heating of the CC. Now if you install a standard SBC T-stat into an LT1 that's a different story. LT1 t-stats are two way while SBC are one way. There's a diff.
And if you have coolant coming out of the back of the engine check the coolant transfer tube at the back of the heads for leaks..
Matt