I would ditch the cats/y-pipe for 2 1/4" pipes off of manifolds following stock routing then a 2-into-1 collector from summit 3" single pipe back to muff and 3" tail pipe on a 5th ave/dippy/fury.

this works bestest IMHO,the headers (flowtech) will be the one fits all A-body header that has the 3 tubes down under the tierods. APITA to install and have no ground clearance unless you crank up the t-bars,have to strip/plug off all smog stuff. (which I just block at the manifolds or remove it all with headers) did I mention fitment sux?

I would go with a stock 4-v intake/q-jet,a999 HS-LU trans,2.94 gear,235/75/15 tires. if this gets 19 mpg in my 85 truck it should be great in a 5th ave with better ride comfort.