My opinion is a sbc 350 in a gmc is a crime. There is nothing wrong with keeping the original 270 6 cylinder or the original 336 gmc(pontiac) v8. I don't think $4500 is all that expensive for what your mechanic wants to do, but I would suspect it will be $8500 by the time he is done.
Get it running first.
Work on the brakes next
then work on the electrical.
