Authored by Bob Tiley, here is his E-Mail link, last revised on July 31, 1997
PLEASE NOTE; Any time working on a raised car, it is mandatory that jack stands are used. If removing the front suspension, a spring compressor is required in order for the work to be done safely.
All attempts have been made to provide correct information here, however,
having personally done this swap on a 1969 model, some other years may
involve slightly different parts. Any input is welcome!
There is nothing like opening the hood of a nice Chevelle and seeing those fat valve covers of a big block. There are many advantages to making the switch from small block to big block power like the monster torque available under your right foot and that unmistakable deep rumble.
Its not that difficult, thanks to the guys at Chevrolet everything is a bolt in, but you must first scout around for all the parts you will need. If you get a long block, like I did this last time, you will need quite a list of parts, most of which are listed below:
Some parts from the small block can be reused like the distributor, pulley on the water pump, power steering pump, alternator, starter (most), and the flywheel or flexplate if installing a 396 or 427.
Lets start with dropping in the engine. Frame mounts were different on 307 and 6 cylinder cars, but the mounts from a 350 will work. Frame mounts are no longer available from GM so you will need to find these on your own. Purchase new rubber or solid engine mounts from a local parts store for your specific car with a big block. If you are installing a 454 or 502 engine they are externally balanced and require a special harmonic balancer and flywheel or flexplate which are weighted to match the engine. It is easier to decide on headers or exhaust manifolds before the engine is in place since they can be installed with the engine. Headers with primary tubes larger than 2" diameter may need some clearancing at the steering shaft. Larger diameter exhaust may need to be installed since these big blocks really need dual 2 1/2" pipes to breathe.
Your existing transmission will bolt up the same as it did to the small block and the z-bar and linkage for a manual transmission will all fit as it did before. Most small block starters will bolt up as long as you have not changed the diameter of the flywheel. Some blocks have starter mounting holes straight across from each other and others are on a diagonal, the starter must match or have all three holes.
Water pumps come in a short and long configuration, Chevy made the change in 1969 to the long water pump. Brackets seem to be a little less expensive to get for the long water pump setup but maybe it's only because I am more familiar with them. The brackets for the alternator are readily available in chrome from any speed shop or in steel from GM. The easiest power steering brackets to use with a long water pump are those from a mid 70's to mid 80's pick-up with the 454 and consists of 2 brackets and a small spacer, they are still available from GM. They will allow you to reuse your 69 to 72 small block steering pump. Most Chevelle parts houses sell reproduction brackets for short water pump applications. The balancer and bottom pulley for the new big block are necessary, but many small block water pump pulleys line up properly.
Don't go too tall with the intake manifold, anything taller than an Edelbrock Performer RPM, unless you have a cowl induction or domed hood for clearance, won't fit. The accelerator cable for the big block is longer and is required along with a bracket to hold it. Fuel pumps are a simple bolt on for the mechanical pump and your old fuel pump pushrod can be reused if needed. A new line from the fuel pump to the carb will need to be made from steel tube or braided hose. If you've gotten this far it won't be long until you are ready to fire up that big boy under the hood only some fittings, belts, plugs, wires and hoses and you should be ready to go. Don't forget to install new front coil springs unless you want to bottom out that nice exhaust and crossmember over every bump.
Notes About the New Mark V and Mark VI Engines:
Remember that the new Mark V big blocks do not have a boss to mount the z-bar pivot ball if using a manual transmission. A bracket can be made to bolt on to the bellhousing to hold a welded on ball stud if needed. Mark V big blocks also do not have provisions for a mechanical fuel pumps and you will need to mount an electric pump near the fuel tank. The latest Mark VI engines have fixed both of these inconveniences. With both of these engines the supplied oil pan will not fit the Chevelle chassis, Moroso and Apple Chevrolet in York, PA (Area code 717) both offer oil pans to fit the Chevelle. Even with these oil pans early Chevelle (64 to 67) steering linkage will hit, so some minor clearancing must be added with a hammer.