1998 360 Magnum or 1965 273 cu in

............... ^^What he said ^^..............

As to exhaust manifolds; forget the 273 stuff. For a few bucks you can have some way better flowing big-port stuff.
Theoretically you could down-cam that 5.9M a lil, and make some big low-rpm torque and run some really small rear gears, and you sure as heck wouldn't need a 4-speed.
But hey, this is one place you could use the 833od, and run some bigger than usual gears, and then cut them back down to size with overdrive.

Here is my thinking on that;
The A833od has ratios of 3.09-1.67-1.00-.73od
The roadgears with 3.23s are; 9.98-5.39-3.23-2.36
With small tires around 25.5 tall (~80" roll-out), 65=2020rpm
Starter gear is 9.98 so lots of blast-off gear, and
Second is 32mph @2280 for a nice around-town cruise. If you need more power (doubtful) you can get it in First at 32=4200
So that sets the tone for the engine.
The stock cam is small enough that you can use the entire engine as is; just slam it in, hook it up, and go.
However, lol;
the 5.9M is not known for getting stellar fuel-economy. So; since your car is light enough that it will have no problem moving down the road at 65=2020;
AND the gears are all useful;
you could, if fuel-economy is/or becomes, an issue;
that stock 5.9M has enough room in the Scr for you to play with smaller cam specs;
which will give up nothing in your application, and probably make it even more fun to drive. Sorry about that long run-on sentence.