I'm a poor one to give advice on something I've never done myself.

The link below shows the engine (not an LS1) resting on a block of 2" wood on top of the TCI crossmember.
Beside of it is a picture of the engine mount welded to the frame.
The builder elected to remove the old crossmember rivets to shift the crossmember forward, for better access to the R&P connections, then bolted in place.
The problem with moving the old crossmember forward is that you have to rework the mounting for the original radiator frame that sits on top of this cross member. It's important to do this correctly because the whole front section (fenders, grille, panels etc.) have to line up perfectly with the holes in the radiator frame.

https://www.1952chevytruck.com/1949-53_chevy_truck_tci_ifs_suspension.htm

And for the big engine and transmission, you blowout the fire wall.

https://ls1tech.com/forums/conversions-hybrids/939949-ls1-into-51-chevy-pick-up.html

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."