The LS1 does not use a standard SBC bellhousing bolt pattern. Hence, the T56 from the 1998 - 2002 f-body will not bolt to the 4BT using the Chevy tranny adapter from the P30 van. The T56 from a 1993 - 1997 LT-1 equipped car uses a standard SBC bellhousing pattern, BUT it uses a proprietary flywheel and a pull type clutch. It also has a very shallow bellhousing. While the LT1 T56 will bolt to the 4BT Chevy adapter, it will be quite a challenge to get the clutch operational. Adapters are available to mate the LS1 T56 to a SBC bellhousing. Using one of these adapters will allow you to bolt the LS1 T56 to the 4BT Chevy adapter, but you will still have to sort out the clutch.
The 1994 - 2002 f-body T56 ratios are: 2.66/1.78/1.30/1.00/.74/.50. The Ford ZF S5-42 ratios are: 4.14/2.37/1.42/1.0/.77. With the T56, you will need to use a rear axle ratio of 5.50:1 in order to have the same mechanical advantage as the Ford ZF tranny using a 3.55 rear axle gear. The 9" Ford does have a 5.50 ratio ring & pinion available, but the gears have 33 & 6 teeth respectively making it one of the weaker gear sets. With a 5.50 axle ratio, you will be forced to tow in 6th gear. The T56 6th gear is not designed to handle towing.
While the LS1 T56 could probably be made to work, I don't think it is a good choice for a 4BT.
Mike