[The Ape Man]

One way is to remove the existing sbc frame mounts and mount some flat 1/4" X 5" X 10" (or so) steel plate stock (don't quote me on the plate size it's been 14 years since I last did this). Drill the steel to match the car's original frame mount holes and bolt it in. The 2 plates will allow you to mount a set of Cadillac frame mounts further foward on the frame. The Cad mounts are way forward from chevy types. You can then lower in a test Cadillac engine block with 425 RWD mounts already hooked to a pair of frame mounts. Hooking to the transmission bellhousing will give you the proper position. A level across the valve covers helps. Trying to do it without the transmission in place requires very careful measurements and is asking for vibration problems. Position everything, mark the plates for the frame mount holes, drill and install. If your transmission has an SBC only bolt pattern then you will either need an adaptor for BOP or another transmission. Note: Use of a fuel pump other than the mechanical block mounted type will help access.
Last time I used this method was in a 1982 TransAm. I replaced the 3-yo-5 with a 500 Cadillac. The steering column had to be moved over at the floor about 1/2" due to interference with the exhaust manifold. I also landed up having to change windshield wiper motor styles. The thing fit pretty well but oil pan fabrication was a major pain. I was happy to find that the Cad engine was not all that much heavier than the SBC. The front springs were left alone and the car just sat a little lower.