Ls4 engine swap
The LS4 uses the GM Metric bellhousing pattern and doesn't share any bolt holes or alignment dowel location with the the SBC bellhousing pattern used with the LS(x) RWD engine family.
You can install a RWD LS(X) based engine, but you will need an adapter plate that keeps the crankshaft flange at the same depth into the 4T65e-hd. You will need to use the LS4 flex plate and starter setup as well.
Since the LS4 block is about 3/16" longer (at the bellhousing side to clear the bolts on the rear camshaft cover) and the crankshaft is 3mm shorter on that end, the adapter plate needs to be around 5/16" thick.
One thing to consider, to get a RWD block to clear the 4T65e-hd, you have to cut the RWD starter pad portion of the block off. This will ruin the block from being able to be installed in a typical RWD fashion in the future, so it is something to consider.
Since the backside of the crankshaft is 3mm longer, everything that bolts to the engine would have its position shifted in the engine bay 3mm (.118") to the passenger side. So you may need to slot some engine mount holes, but this isn't much of an offset, so it shouldn't be a deal breaker. Since the engine placement will move, but not the transmission, the exhaust crossover will likely have reduced clearance to the transmission bellhousing flange, same with the down pipe going under the chassis.
On the balancer side, the RWD crankshaft is 10mm (.394") longer. The LS4 crankshaft is recessed slightly (I forget the dimension - around 1/8") in the LS4 balancer, so the critical extra length is around 1/4". The RWD crankshaft will poke through the outer side of the balancer, but you can use a sleeve with 1/4" wall thickness and about 5/16" in length to enable the crankshaft bolt to properly clamp the balancer.
You can install a RWD LS(X) based engine, but you will need an adapter plate that keeps the crankshaft flange at the same depth into the 4T65e-hd. You will need to use the LS4 flex plate and starter setup as well.
Since the LS4 block is about 3/16" longer (at the bellhousing side to clear the bolts on the rear camshaft cover) and the crankshaft is 3mm shorter on that end, the adapter plate needs to be around 5/16" thick.
One thing to consider, to get a RWD block to clear the 4T65e-hd, you have to cut the RWD starter pad portion of the block off. This will ruin the block from being able to be installed in a typical RWD fashion in the future, so it is something to consider.
Since the backside of the crankshaft is 3mm longer, everything that bolts to the engine would have its position shifted in the engine bay 3mm (.118") to the passenger side. So you may need to slot some engine mount holes, but this isn't much of an offset, so it shouldn't be a deal breaker. Since the engine placement will move, but not the transmission, the exhaust crossover will likely have reduced clearance to the transmission bellhousing flange, same with the down pipe going under the chassis.
On the balancer side, the RWD crankshaft is 10mm (.394") longer. The LS4 crankshaft is recessed slightly (I forget the dimension - around 1/8") in the LS4 balancer, so the critical extra length is around 1/4". The RWD crankshaft will poke through the outer side of the balancer, but you can use a sleeve with 1/4" wall thickness and about 5/16" in length to enable the crankshaft bolt to properly clamp the balancer.
realistically your best option is to find a junkyard ls4, make a good friend at a machine shop that is very good, and have them resleeve (because we have aluminum blocks they are sleeved for pistons to"rest" on) for the displacement you desire (i am not exactly sure if the ls4 are dry or wet sleeved)
best
best
