[66 C10 383]

I'm doing the same swap and yeah, it can be confusing. There are several ways you can go about this.

Some decisions to affect the final choice are, do you want to run electric fans or the stock LS mechanical fan. The engine needs to be back a little farther if using the mechanical fan.

Some guys can also place the engine so they can reuse the stock driveshaft. May have to redrill the frame to reposition the tranny crossmember.

Some frame perch/mount options will allow you to run the stock 5.3 ac compressor in the lower position. These can be retained without cutting the frame although the lines from the compressor get mighty tight in there.

Depending on your choices, you can use the stock frame perches and mounts by using adapter brackets or the slider brackets. Some of these will allow the LS bellhousing to be in the stock bellhousing location. They also let you move the engine forward different amounts by varying amounts depending on which adapter brackets you choose. Depending on your fabrication skills, the can also be made yourself.

You can also purchase aftermarket frame mounts if you want. There are several different companies who sell these. Some use the newer style 3 bolt Chevy motor mounts (with the bolts that go thru the mount rather than under the mount like stock).

Early Classic also sell a setup that uses the clamshell style motor mounts. These require you to narrow the top frame rail a small amount for the frame perches to sit properly (when the early 70's trucks came out with the big blocks, the frame was narrowed a small amount for exhaust manifold clearance so this small amount of narrowing shouldn't be much concern safety wise.) These mounts, from what I've been told, move the engine forward which requires running an electric fan due to lack of space for the mechanical fan.

I know this isn't/wasn/t the definitive "use these" answer you might have been looking for. Use it kinda as a guide for your choice. Your requirements will determine the final option.

Myself, I'm looking to shoot for the stock location bellhousing location for a farther rearward engine. That will allow me to run either a stock mechanical fan or electric fans. So I'm going to use the stock perches in the stock rear frame holes and an adapter bracket.