I dropped a hei on ours yrs ago...recurved it...
Better starting, less maintaince, less problems when cleaning / high press hoseing the engine down, seemed(?) to pull smoother at higher rpm...This is usually to opening the plug gaps up, (increased resistance across the gap) supplying higher voltage and better healther stronger spark enabling increased assurance of that cylinder exploding the mixture every time.
Make sure u also up grade the HT to 8mm and keep HT leads separated.
Gas mileage...dont believe all the marketing propaganda m8, thu it can make a large improvement..but no more than if your orginal is stuffed and fixed that.
If so Should I buy a new Distributor or just change the guts?
Check yours for wear in the shaft, shaft bushings, centrifical mechanism bushings, centifical weight bushs...If those are worn, NOTHING will change anything unless those are replaced/rebuilt.
If a street car or street/strip, depending on how much money u have to waste..dont bother with expensive dizzies, u will not get any more than u would out of a HEI...or even a well set up stock pionts dizzy...other than bragging/noting rights.