As far as the engine swap, it really shouldn't be difficult at all. First, your Skylark is basically the same chassis as a similar-era Olds Cutlass, which was offered with a 455 in '68 and '69 (in Hurst/Olds models), and then as an option for the standard Cutlass from '70-'72. Since the 350 and 455 Olds use the same engine mounts, even a set from a 350 Cutlass should work to mount the 455 in your Skylark. The proper frame mounting holes are probably already drilled in your engine crossmember as well. You also mentioned the need for a new transmission crossmember, probably because you figure this will be necessary to mount the TH400. However, on most A-body GM cars of that era, the same transmission crossmember is used with all transmissions-it's just slid back for the larger trans. Again, the holes in the frame to mount the trans crossmember in the TH400 position should already be there, since some Skylarks-like the GS 400-could be had with the larger Hydramatic box. Some Buick/Olds/Pontiac applications from that era used transmission crossmembers that had rubber isolators on the ends to dampen vibrations transferred to the chassis, and if your car has this, you may want to find the one-piece Chevelle-style tubular crossmember that does not have isolators. After 30-some-odd years, the rubber bushings may not deal well with the monster torque of a 455 and a heavy foot. Note also that you will need a shorter driveshaft that uses the larger TH400-style slip yoke; your existing shaft could be modified by a competent shop to work.
Pontiac Big-Block?From what I was told a while ago, the Pontiac V-8s are all the same block from the 350 to the 455. Yet I keep seeing listings on eBay claiming the 400 is a big-block. Which is correct?Carl McCoyVia the Internet
You were right the first time, Carl. The Pontiac V-8 that was first introduced for the '55 model year has come to be known as the "traditional" Pontiac V-8 among enthusiasts, to differentiate it from "corporate" V-8s (read: Chevy) found in many Pontiac vehicles, a GM practice that began in the late '70s. All traditional Pontiac V-8s are based on the same block design, from the 326 up to the 455. This is not to say that all of these engines actually use the same block-they don't. They do, however, share external dimensions. The 421/428/455 versions are called "large-main" engines, referring to the 3.25-inch main journals these engines have as opposed to the 3-inch mains of the 400 and smaller engines.
Engine swapping is fairly easy among Pontiac vehicles, with one primary obstacle in the motor-mount bosses-early motors have two holes per side while '70-later have five, a move made to accommodate the second-generation Firebird's engine positioning. Some oddball engines circa '73-'74 seem to have three holes, as the other two bosses were not drilled for some reason. Nunzi's Automotive in Brooklyn, New York, offers adapter brackets for those wishing to mount a pre-'70 engine in a later Firebird under part number 8070.
Score Or Scrap?I have a set of SBC heads with the casting number 10125320. How do these stack up against the Vortec heads? What size combustion chambers do these have?Greg GortonWillis, TX
The casting number you listed is for a set of '94-'96 LT1 cast-iron heads. These would have been used on the 260hp version of the 5.7L LT1 found in the Caprice, Impala SS, Buick Roadmaster, and rear-drive Cadillac. As far as the port design, the Vortec's ports were based on the Gen II LT1 design, so they are very similar. Most LT1 heads have 64cc combustion chambers used in conjunction with the flat-top pistons in the 5.7L short-block.