Most people are not willing to modify the doghouse area of the cab for transmission clearance. One pretty slick way to do that is to use a wheelbarrow tub for the firewall modification and trim it to fit the opening you create in the cab/floorboard area. It's usually simpler to modify the center crossmember to allow a lower engine mounting position. Also, unless you fabricate a crossmember to support mounts in the center engine position, there's a risk of breaking the bellhousing. Don't try to support the engine/trans assembly at the ends without using the mid-block mounting position.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!