I share your vision & hope someone responds to your post so I can get some
idea's. (2) projects need to be completed &then I Will build a diesel car
too.

Hotrodders do this all the time - it's considered the norm. Read any
Hotrd magazine and you'll find more suppliers for this kind of
conversion than you can shake a brake line at. ;)

Mopar sells spring perches as a seperate item. You chop off the original
spring perches from your modern axle, have it shortened to fit in the
old car, then weld on the new ones - pretty slick. You'll probably have
to have the spring packs made new since I doubt the old ones are worth a
damn anymore. They tend to lose their spring and get too bouncey when
they get to be about 50 years old. Last thing you need is a heavy steel
body on a fast/modern driveline bouncing all over the freeway at 75 mph
- especially if you don't re-engineer the steering or install a new
hotrod front-end setup. There are several places that will make up a set
of new springs for around $100/wheel at any arch & length you need.

The nice thing about a modern axle is the wide range of options for disc
brakes. WAY better than the original drums. My Willys Wagon 4x4 will
have new Land Cruiser dual plane rotors & calipers on the front - only
about 1,000 times better than the old drums.

BTW, that sounds like a fun car to drive. Always wanted an old Buick or
Olds to fix up. Just far enough outside the norm that you probably won't
see another one just like it at the local cruise. Nothing wrong with a
57 Chevy (for example - great cars) but

Good luck,
 - Jeff G
http://jeffgross.com/willys

So, interesing project , go for it if that's what you want. I'm interested
to know what year this old Buick is and model number. Some of your throw
away parts may be usable by other folks.
Thanks, JR in NC, '42 Buick

Something to keep in mind if you propose to put in a "big 6-cylinder
diesel" (presumably the turbocharged Cummins as found in Dodge pickups
and the like) is the weight of the new engine.  Although the Buick
straight-8 was no skinny mini,  you'd still be adding four or five
hundred pounds.  This has implications for the front suspension and
also the chassis.  I'd contemplate the Duramax V8's dimensions as it is
quite a bit lighter, were I doing a diesel retrofit.

And when converting to a USAmerican pickup-truck diesel, you'd have to
get the transmission that comes with it, or else retrofit a manual
meant to deal with an engine that redlines about where life gets fun
for a gasser -- and capable of producing 500 or so foot-pounds of
torque.

Or consider whether a small-block Chevy gas V8 would fit -- it'd sure
make life cheaper and easier.  The reason the rod and custom crowd
loves these is not that they're the greatest engines ever, but they're
good, and they're small and light and simple by the standards of their
displacement/power class, and cores, crate motors, parts, upgrades,
etc. are thick upon the ground.

Note also that there's quite a bit of interest and parts availability
for these old cars, including some options intermediate between a
full-on custom and a restoration.  Would you believe that a Straight
Eight crate motor is available (as are parts) and so is an adapter that
lets you use the ubiquitous GM TH700R4 automatic with it?

Rear axles are sort of a free variable if you bring money.  The
nine-inch Ford is particularly though not exclusively available, and
excellent, for these purposes.  As long as we're opening Pandora's
toolbox, four-wheel disc brakes would be awfully tempting.

Or you could restore it.   Or sell it to someone who would, or who
would use if for parts.  What sort of shape is it in otherwise?

Anyway, all things are possible with mechanical skills and money,
preferably both, so I reckon you've got a big strategic decision to
make:  resto vs. discreet upgrades vs. major customization.

Sounds like either a fun project or a benefit to restorers.  Just be
clear in your mind about what you'd like the end product to be, before
doing anything irreversible.  (And yes, preserving the correct
powertrain is a great move, if you've got the space.)

Best of luck,
--Joe