No, the MGC and AH blocks are not interchangable. The MGC actually shares very little with the AH3000, so modified was the C-series block between what went into the AH3000 and what finally went into the MGC.
For instance, the AH3000 has 5 main bearing, the MGC 7. And there are countless other differences.
The MGC did share its engine with the Austin 3 Litre sedan, but that car wasn't sold here and I believe it had a lower compression ratio through different pistons.