As corny as the old adage is, in most instances it's true: There's no replacement for displacement. And these days, with the availability of stout mail-order crate engines, the cost of buying a brutish big-block is now within the budget of many gearheads.
Case in point: GM Performance Parts' burly 502/502 big-block (PN 12371171) that can be had for about $6,000. In the Sept. '97 "Crate Engine Shootout!" story we tested the 502/502 and were giddy to see 513 hp and 555.5 lb-ft of torque. However, even though a crate engine's price may be lower than what you'd spend building a similarly performing mill, we wondered what hassle and expense would be required to shoehorn such a fat-block into a traditional musclecar.
We'd always heard that a big-block Chevy was an easy swap into a small-block car--but is it true? With the 502 and an A-body on hand, we decided to find out for ourselves and share with you the details and the costs. As usual, CC's much-abused Cheap Street Chevelle was the guinea pig, but much of the info will apply to most '68-'72 GM A-bodies or GM fullsize barges.
After jerking the small-block and test-fitting the 502, the first thing we learned was that the 502/502 comes with a six-quart truck-style oil pan that interferes with the engine bay in our '70 Chevelle. We replaced it with a special Milodon oil pan that merges the correct old-car-shape to fit the chassis with the new-car-type pan rails that fit the new Gen 5/Gen 6 big-blocks and their one-piece rear-main seals and revised gaskets. The Milodon oil pan's unique design has a large seven-quart oil capacity for reliable oiling during racing yet is a low-profile design to avoid ground-scraping woes. Often, you can reuse the 502's stock one-piece rubber oil pan gasket, but if not, companies such as Fel-Pro offer a gasket under PN OS34407R.
The Milodon oil pan (PN 30955) requires a special oil pick-up tube assembly (PN 18301) to be properly oriented within the new oil pan's sump area. | 
If your '68-'72 GM A-body originally came with a 350 small-block, you can retain the original engine frame mounts. If it had a 307 V-8, you'll need to switch to 350/396/454-style engine frame mounts (GM PN 3980711 left, 3980712 right), which should also work on non-Chevy A-bodies. On the engine side you'll need to upgrade to new GM big-block engine motor mounts (PN 03962748). | 
We installed a new Milodon 160-degree-F thermostat and then bolted on a reproduction big-block-style thermostat housing (PN 26016) from OPG. Rather than reusing the crusty old bolts from our small-block, we opted for an ARP big-block bolt kit (PN 535-9501). |
The 502/502 comes with an aluminum water pump, but it's designed for Reverse rotation and would require switching to modern-style serpentine belt-type pulleys and brackets that would be a bit pricey. We installed a new Edelbrock long-style aluminum water pump (PN 8851) that accepts traditional early type pulleys, brackets, and hoses. | 
New crank and water pump pulleys are available from OPG. We opted for LS6/L78-style deep-groove crank (PN SL39119) and water pump (PN SL39414) pulleys to avoid any high rpm belt-throwing mishaps. A 15450 fan belt fit perfectly. | 
The small-block alternator brackets won't work with the big-block, so we installed OPG's big-block alternator bracket set (PN FL8502). Our old alternator already had an aftermarket deep-groove aluminum pulley, but if you want one of OPG's deep-groove pulleys, order PN SL39120. |
We found out the hard way that small- and big-block (right) power-steering pumps looked the same, but had slightly different lines exiting the back. To use OPG's factory-type big-block brackets (PN SFL8504) we also needed to upgrade to a new (expensive, ouch!) big-block power-steering pump (PN TRI6165) from OPG. | 
A big-block (including Gen 5/Gen 6 big-blocks) fits into the engine bay of a GM A-body without much hassle, but getting headers to fit was the real trick. Hedman offers two varieties of 13/4-inch headers (full length and 3/4-length) that work great. We opted for Hedman's 3/4-length headers (PN 68610) because they're easier to install, tuck up tighter, and provide added ground clearance. The headers can also be ordered with a trick hi-temp coating for an added price, but gaskets and bolts were included free. Before installing the engine, we test fit the headers to make sure they cleared the gear-reduction starter (included with the 502), motor mounts, and oil pan. If you prefer, stock-type manifolds are available from OPG. | 
We learned that the passenger-side header must be installed from underneath the car. Have one guy slip the headers in from below while another installs the gasket and bolts (included) from above. |