I have done them both ways. I converted my 56 to a 350 engine in 2000 using the stock V-8 front mounts and the rear bellhousing mounts.
That is the factory configuration and works well. I have also used the aftermarket cross-member for center mount engines with both the bellhousing mounts or the single rear mount used for later model manual and automatic transmissions. What you don't want to do is use the "bird leg" front mounts with the rear center mount. There is too much span front to rear with that configuration. That may be the horror story you have heard about. I encountered one that the only thing that was keeping the engine and transmission in place was a solid welded exhaust system.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine