they are do-able..
I saw a 4x4 GMc with a 455 Pontiac motor at a truck show last summer..
The guy used firebird motor mounts and had to use a flat plate to extend them back to fit the crossmember..Pontiac,Olds,and Buick,also Caddy motors have the motor mounts made into the block about 6" or so more foreward than the Chevy motors..all you need to do is use a heavy plate bolted where the Chevy mounts once bolted to the crossmember to move it forward enough to line up..this truck had a Pontiac TH400 with the chevy 4x4 output shaft swapped in..guy said he originally used an adapter,but the Chevy tranny croaked,and he found a Pontiac one cheaper..
I've seen others use a stock 6 cylinder motor mount bracket,and they use the passenger side on the drivers side to make the mounts work..others made custom tubular crossmembers instead..many ways of getting around the motor mount dilema..one good thing is Pontiac and Olds oil pans fit,they already have a rear sump..Caddy motors need an Eldorado pan with a rear sump to fit..
Just about any GM V-8 will work in a 73-87 full sized truck..we put a 403 Olds motor in a 75 Chevy 2wd 1/2 ton at the junkyard...used the Olds TH400,had to make the motor mounts fit,wasn't that big a deal(we used factory diesel 350 Olds motor mounts and brackets from a junker 79 GMC,it was basically a bolt in deal!)..but moving the battery,fuel lines,and cables was a pain--Olds has the fuel pump and starter on the drivers side!..we also used the exhaust off the diesel Olds 350,it worked perfect...
I've seen 455 Buicks in GM trucks,and one guy even had an older Caddy motor in his 2wd tow rig,I think it was a 429 motor..wasn't the usual 472 or 500,it was the older one..had mucho tourque!..he bought the motor from us at the junkyard from a '64 Limo..smoked the tires off it pulling a car trailer with a heavy car on it like nothing!..
