Here's a little more info on the specs for the 6th generation Riviera ('79-'85). It appears to have been available with the 307" Olds V-8 engine in addition to the Buick V-6 and not the 350 V-8..
1981 saw the Turbo renamedT-Type and the demise of the 350 engine in favor of the Oldsmobile-built 307 cu in (5.0 L) with 140 hp (104 kW) (phased in during the 1980 MY). The standard engine was now Buick's 125 hp (93 kW) 252 cu in (4.1 L) V6, and a new option was an Oldsmobile diesel engine with a mere 105 hp (78 kW) offered through 1985. 1982 also saw the first-ever Riviera convertible, although relatively few were built, owing to a high price - US$23,944 ($72,608 in 2022 dollars [13]).[40] The Riviera convertible was available in only two color choices-white or red firemist with the only interior color of red leather. A twin-turbocharged Riviera convertible was chosen to be the pace car at the 1983 Indianapolis 500, tuned to produce 410 hp (306 kW).[42] Most convertible Rivieras had the V8 engine, which saw an increase in rated SAE net HP to 150 for both convertibles and coupes fitted with it from 1982 through the 1985 model year.