I don't have any direct engine swap experience with LBCs.
But I have a couple of opinions to offer.
Every GT6/Spit conversion I'm familiar with, (I've seen 3 or
4) has been executed the same way. The spit body was moved
over to the GT6 frame and the bonnet was retained from the
GT6 (necessary for clearance of the carbs).
I have a friend who removed the Triumph running gear from
his TVR 2500M, and replaced it with the Buick 215. Was
really a nicely down job. But he spend MANY months of work
to get it down, so the motor mounts were in a correct
location and the hood would close with out cutting a hole in
it and the drive line angle worked.
He had to move only on frame support in the box frame ( a
vertical member between the bottom and top horizontal
members). He was able to remove a lot of weight from the
front end. As a result the then had to redo the front
suspension, getting new shorter springs to bring the
front of the car back down to a reasonable ride height. But
there are some post swap headaches that he now gets to deal
with, like the headers are totally custom (in his case,
hand-built, because he was unemployed at the time), custom
radiator, and service headaches like having to remove the
headers to do anything, even removing the starter requires
this. He also went right on back to handle the extra
horsepower, replacing the transmission with a Borg-Warner,
and the rear-end with a Corvette limited slip (he considered
a Jag unit, but they were to expensive). A lot of work,
but man does that car move. I suppose it worked out to well
for him though, now he is thinking about putting a 215 in
his recently aquired TVR 3000S.