not accelerating

I have an 88 C-1500 w/ a newer Jasper 350, 700R4 and a new torque converter. When I go to accelerate the engine will drop @500 rpms and them slowly climb. It is also eating gas like no tommorrow. I have gone over everything and have determined that is probalby the catalytic converter. The previous engine had a blown head and head gasket and the exhaust shop said that antifreeze and fuel co0uld have got into the cat. I need anybody out there with any help or being able to confirm my suspicion.

Connect a vacuum guage to the manifold vacuum suply. Snap the throttle and the vacuum should drop and rise (in that order) almost instantly. Hold the RPM up to around 2,000-3,000 and watch the vacuum guage. If it steadily falls, you have plugged exhaust.