"...the 428 will bolt in to a G body frame with the factory 301 mounts..."
Not without adapters. The 428's do not have motor mount holes in the correct locations.
https://butlerperformance.com/i-2445...tegory:1393557
"...don't know block/heads etc. Never been down this road..."
Don't buy a 500557 or 568557 casting number block. Don't buy heads with press-in rocker studs. By 1974 & later heads, such as #46, 5C, & 6X. They have screw-in studs & factory hardened valve seats, for unleaded gas. The 5C & 6X heads have larger 2.11 intake valves. The 6X heads are very popular on 455 engines, and are quite common. The '71-up D-port 455 heads have big combustion chambers & will not make enuff compression for a smaller engine. The '75-'76 6H 455 heads have 124cc chambers, and are especially low compression.