--What Gig said and other things.. as your wallet allows... ).
--Lose the 140 pounds of bumpers..... or modify to lessen the weight, or go the pre-75 stainless bumpers... again.. as your wallet allows... the weight from the bumpers gone helps with the GVWeight.
--Suspension parts are the same as earlier... I have a 77.. going to a 74 'feel' as we speak. ( a thread on here as I progress.. ) There were/are aluminum spacers under the springs for DOT required ride height..., every car a bit different. But you can leave the springs and such stock... or start lowering. Spacers out will do that to start.... lower/shorter springs will do it more, either cutting the stocks, or installing aftermarket. With my Spider, I went with IAP red lower springs.... Lower than stock, looks great, rides 'better, a bit stiffer, and new.
-- Any work you can do with the cylinder head will improve performance... From making sure the cam's are on time marks, replace the cams with performance cams, mill the head, tougher springs for the valves bring higher rev's, big valves, etc... as your wallet allows..

--Various things to do with the ignition/distrib system... replace with electronic, pointless, variations.
-- Fuel delivery improvements... Modify Spica system to changes with engine mods... If you go carbs, there are a ton of threads on what to do with them.
--Most of these changes can be done to make the car more reliable as well as perform better. A better breathing beast that has better 'shoes' to run in.... which means tires too... lots of options too...