I think you are on the right track by drilling the frame and the front plate. I ran into this issue installing a 235 from an early 60's car in a 59 truck a few years ago. The car engine only had provisions for side mounts. I don't remember what mount I used at the time but as far as I know the truck is still running. You could probably use a 54 mount or something like a 350 transmission mount.
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