Here's the brackets that you need -
https://www.classicmuscle.com/c10-bl...mounting-brkts
If you need more help -
http://67-72chevytrucks.com/vboard/s...d.php?t=422411
There are many companies the sell this bracket kit, just search "73-74 Chevy Grille Brackets", and choose whichever vendor you'd like to buy from.
BTW if your 78 grille is in good shape, I'd keep the Blazer looking like a 78.