i've been messing with a similar combo in one of my cars for several years trying to find a sweet spot; still searching. my engine is a stock 440 and basically your putting together a physically smaller 426-440 wedge. my car has a midland ross power brake booster and it just doesn't like anything 15" of idle vacuum or less. the booster does fine with 16"-17" of vacuum at idle. the cams I've tried thru the years are the stock magnum, comp cams 21-305-4, 272/.455 mopar, hughs/engle 23/30 and summit 6401. the comp cams cam will make vacuum but runs out of steam rather quickly (probably due to 106cl installed). the 272 mopar looses vacuum, summit and engle even more. going from a true 1.5 rocker to a true 1.6 rocker will loose 1" of vacuum. the faster the lobe profile with the same duration will loose some vaccum. currently i have the stock mopar cam (actually a sealed power cs661, same as stock) in the engine and vacuum is up, brakes work, drives nicely, but performance has been better. that 275hl is going to give you about .560" lift at the valve, about 15 degrees overlap at .050" (something i'd never do with stock parts).

if you insist on the 275 type cam i'd suggest an xe274 ground on something like a 113 or 114lsa. the 275 is a 274 with the added area under the curve that a 274 has with 1.6 rocker. with all the tappet/cam problems people are having i'd error towards the gentler side. the faster the lobe, the higher the rocker ratio the more spring you need. the more spring you have can kill a hydraulic tappet and cause lobe failure.