85 k10, took off catalytic converter now backfiring

[gbnbrb123]

Okay, so I am a little new to the exhaust world but figured I had some knowledge of the subject matter. I removed most of my exhaust because of corrosion and rust and literally falling to pieces. The k10 is stock with a 305, the only pipes left on the vehicle is the y pipe. After taking the catalytic converter off the truck has started backfiring and barely starting at times, while previously it was running fine, at least I thought so. I went ahead and replaced the spark plugs due to how bad it was running and fuel filter, but no change, time for them anyway. I also had put in a new gas tank prior to everything along with a new fuel pump and everything seemed to be running fine at that time. My plan was to run straight pipes with maybe a magnaflow muffler with exit behind front door but was curious if that would make any difference in how it was running due to backpressure. My other option is to put a generic 2.5" catalytic converter with piping run to a muffler. Curious as to what you guys think are the best options and why the truck is running so rough.
Thanks, Nick