swappin a 292 for a 350

['68OrangeSunshine]

So you want to keep the SM420, drop in a 292 L6 and add a power brake booster in a '67 C/20?
It should all drop in fine. That stuff was all optionable when the truck was new. If you're keeping the stock Rochester Monojet 1-Bbl, there should be no air cleaner issue. If you get fancy with a Holley 2-Bbl or any 4-Bbl on a Clifford or Offy intake, then you may need to get creative with aftermarket air cleaners.
I don't have power brakes on my '68 C/10 w/292, but I have a 12" diameter Moroso round air cleaner with a 3" tall K&N filter element. Carb is an Edelbrock 1404, 500 CFM 4-Bbl on an Offenhauser intake with Clifford headers. Transmission is SM465, straight one- piece propshaft and 3.73 Corp 12-bolt rear end with Eaton Posi.