Good score.

Run the quadrajet.
Methanol injection. http://mysite.verizon.net/res11pgkr/vehicles/buick8485.html

MSD BTM box: http://cgi.ebay.com/ebaymotors/MSD-6BTM-...=item336caa3ea9

Make a downpipe as straight as possible mandrel bends http://cgi.ebay.com/ebaymotors/3-45-180-...=item2314ebdd6c

& 3" would be a good size w/a straight through muffer design.
Just an example: http://cgi.ebay.com/ebaymotors/Thermal-R...=item45fd911377

I personally do not like the sound of a turbo car w/out a muffler & most people I talk with, pretty much agree, an exhaust system w/out a muffler sounds like a$$.

Plus cops will give you a ticket for being too loud w/out a muffler. Also, some tracks will not let you race if you are too loud & have no muffler.

Stock intake modded, or better yet, an aluminum OFFY intake w/a aluminum adaptor plate to make a hole in the center of the plate
& 3 holes drilled & tapped, then bolt down your turbo compressor housing to the adaptor plate.

No reason to block off the secondaries on a quadrajet.

Here is a good read on how to mix & match turbo hot air turbo parts.The Pontiac 301 set-up has larger components for the most part & flows more air.
http://www.turbobuick.com/forums/before-black-non-sfi-tech/274489-all-turbos-not-created-equal.html

HEI will work fine.

I seen an old 1978 Buick Regal @ the street races years ago haul some butt, an old hot air set-up (non intercooled) Same as your set-up.

Anymore questions ask away, glad to help a fellow inliner.
MBHD

12 port SDS EFI