Fitting a 454 big block in my 1969 K20

What I have learned in putting in a big block in my 1969 K20 is that, for whatever reason, some people get the big block into a 1967-72 Chevy/GMC with little to no modification and others have to do alot of modifications. I have my big block in the small block location using my factory small block crossmember, towers and mounts. For me even with a 2" body lift I had major valve cover/firewall clearance problems on both sides of the engine. I stepped up to a 4" body lift as I could not easily move the engine forward at the body shop - though I do plan to drop the body lift back to 2" as soon as I can. Even with a 4" body lift I still had contact with the firewall. Here are pics which shows the modifications I had to do to get plenty of firewall clearance.

Some have to just massage the firewall a little, some have opted to move the engine forward. I don't know exactly how far you need to pull the engine forward but for me it looks like 1 1/2" to 2" would be enough. Using the small block engine crossmember the oil pan clearance on my 1989 1 ton 454 was very close. I did shim the motor mounts slightly to give a little more clearance.

An option is to use a later small block crossmember and mounts. This puts the crossmember in front of the engine. Also helps with header fitment. I am using Dynomax Cermacoated headers, part number 86022, designed for the 67-87 BBC 2WD. The headers for 73-87 BBC 4WD would not work. I did move the crossmember forward about 3/8" or so to give me better oil pan clearance and header clearance for the first header tube on the passenger side. Although I moved the crossmember forward, the location of the engine mounts stayed in the same location by adjusting the bolt hole used to secure the engine to the mount. To make these headers fit I did not have to make any modifications to the driver's side other than I did have to go with a shorter oil filter as this engine has an oil cooler setup which put the larger oil filter in contact with the headers. On the passenger side I had to remove a little bit of frame to give clearance for the first and second header tubes. I would guess I removed about 1/4". I also ended up grinding off a little bit of the engine crossmember tower as it was in contact with the bottom of the first header tube. In the end I had to use the spacers pictured above that I put between the engine and the engine mount bolts to raise the engine a bit on the passenger side and I also raised the drivers side by using spacers about half as big.

The above modifications have me comfortable with the clearance around the oil pan and passenger side headers. Here are the dimension info on the Big Block vs. Small Block measurement differences: Big Block 23.63" length from center motor mount hole to front of motor 7.68" height from center motor mount bolt to top of motor 22.96" wide Small Block 21.78" length from center motor mount hole to front of motor 5.33" height from center motor mount bolt to top of motor 19.68" wide The width and extra height are the problem.

It is all going to be worth it in the end but I can guarantee I won't be putting a big block in my 1970 K5.