philippe - Not so...the #57 Gerhardt Pollard borrowed in practice did run the race driven by the assigned driver, Carl Williams, breaking on the 50th lap . Art drove the #40 Lotus Offenhauser from 12th on the grid to a lowly 31st, breaking on lap 7.
(Also, the McNamara was used in 1970.)
gbl - In '69, Andy Granatellis team used a plethora (well, almost anyway) of combinations of chassis, engines, and drivers with as many as four cars in a race at a time. Mario Andretti was lucky to have the Hawk Ford all year (after the hub failure on the 4wd Lotus at Indy). Not even including the dirt track cars...(sheeesh)...they used different combos: the aforementioned Hawk Ford for Mario; different combinations, seemingly weekly, of Lotus and Gerhardt chassis and Plymouth and Offy engines for,,,(phewwww...) Art, Carl Williams, Greg Weld, Sam Posey, Joe leonard, and Jim Malloy. Oh....I almost forgot, George Follmer did a race or two also.
(If I left anyone out, I claim weariness as a defense. Don't even think about asking me about the dirt track car driver combinations...I ain't goin' there!!! lol)