A very skilled, IMHO, mechanic posted a GMC V-6 into 123 chassis
conversion about five years ago. It took him 9 months to a year and
included enlarging the transmission tunnel, making new motor supports,
changing the drive shaft and inventing a new throttle linkage. He was
very pleased with the result. It wasn't $ expensive but was a large time
Suggest you find a used motor and install like for like.