You guys do know that a 430 looks identicle to a 455 except for the direction the ribs on the valve covers run right?
There's one in the AMT '69 Riv. Not anywhere as nice as the Monogram 455 but it's 1/25 and slightly underscale at that so it's a pretty good size for a 350. Just sand off the valve cover ribs and add new ones, You really should even if you do use the Monogram engine as the 350 has one less rib.
BTW, I just went out to the grage and measured the set of Stage-1 valve covers I have hanging out there and they're about 2 inches longer than a small block Chevy valve cover. Only less than 1/2 an inch wider though. I'm pretty sure that the 350 Buick covers are almost exactly the same size as the Chevy ones. ,, But I haven't had a small Buick engine in ages, just the big block now.
SO ,,,, if it was me ,,, I'd use a Pontiac engine [for the plug area detail in the heads] with the front cover/water pump/distributor fom the Monogram Buick with a modified set of sb Chevy valve covers.
I bet there's a buick expert around here somewhere.
I know a guy on here who's owned a '69 and a '72 Riviera, a '70 Skylark and '72 LeSabre convertibles. a '73 Centurion, took his driver's test in a '74 LeSabre with a 455 in it and is currently in posession of a '68 Electra.