I would verify that the 348 engine you obtain has the mount holes drilled and tapped on the front of the engine to be able to use the factory 59 SB front mount configuration. 348's in cars used the side mounts which would would likely require an aftermarket cross-member type mount for use in a truck. I have done several 6 cyl to SB conversions on TF trucks and all of the front cross members were drilled
for the V-8 mounts. Perhaps the holes are filled with crud but they should be there.
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