Inline 6’s are fun and last. Have a 98 Jeep grand Laredo with a 4.0 has a Howard’s cam and a 62mm throttle body 24lb injectors header and cat back system. It will hold its own.
I've always had at least one inline 6 in my stable somewhere. They run forever if maintained, and sound good stock with no muffler, not even too loud.
I've also got a stack of nos GM service parts "fitted" crate blocks. Someday they'll either get homes, or sold when I die, but I don't think I'll ever be without one for a project.
I'd also like to put a Pontiac super duty I4 into a gbody... likely a grand prix...
They're only about 5" longer than a SBC, or about the fan shroud depth
The fan on the ones I've used always wind up really REALLY close to the radiator. To work on the radiator, even in a squarebody, you need to remove the fan blade prior to the fan shroud (said fan shroud which on sides and bottom is almost non existant) when servicing the radiator.
It'll be tight, but doable like anything else is.