If you want to run a small block Chevy with the stock running gear then as far as I'm concerned the best kit bar none is the Mark's of Australia kit.
It uses an auxilliary bellhousing that sandwiches between the SBC and the stock Toyota bellhousing. The Chevy flywheel, clutch disc and pressure plate are inside the Mark's bellhousing, and the Toyota bellhousing has the throwout parts. An Advanced Adaptors engine mount set is included. Minimal welding is required, for the scab plates for the motor mounts. The kit spaces the SMC ahead of the firewall, so no cutting of the firewall is needed. The Toyota running gear stays in the same location.
The only drawback to the kit is the price. But, it is a well engineered and well made kit, and reduces the frustation level to almost zero. Conversions just don't get as easy as this.
Three things to note: Follow the instructions verbatim when they tell you what clutch and pressure plate to put in. If you go cheap you may wind up replacing it prematurely. And the block hugging headers that Advanced Adaptors sells for this conversion are so tight they will rub on the manual steering box, requiring one tube to be dimpled for clearance. Do yourself a favor and run center dump cast iron rams horns exhaust manifolds instead. I'll eventually convert mine over. Also, the engine sits slightly offset in the engine compartment. This is normal, just like the stock engine.
This kit is sold through Advanced Adaptors.