I think part of whats going on is it isn’t obvious that the engine bay arrows only indicate if the engine fits for FWD (or the most restrictive type) by default, even if you selected longitudinal front engine. It looks like the engine bays are way more restrictive than they actually are.
Some former mod bodies do change going vanilla though, usually for the better, but these had the firewall moved forward more than most bodies and lowered to where macphersons stick through the bonnet (they could just be disabled i guess).
70sUSCoupe_Small can only fit an 800cc I4 in mid transverse configuration (I’m expecting it to be able to carry a 2L I4 or similar), and no V-type engine will ever fit into its engine bay:
Make sure you can’t fit more in transverse, some body layouts are just not that great for longitudinally mounted engines. Always check both directions, especially for mid engined cars.
I re-checked the engine bay size of 70sUSCoupe_Small in mid-engined configuration (both transverse and longitudinal) and it turns out that absolutely nothing fits (except for really tiny inline threes and fours - 800cc to be exact - and even then only when mounted transversely). Front and rear engined is fine, though.
Also, in the open beta, the 10sSuper_Cor_coupe can’t fit any large engines with the mid longitudinal engine placement option selected.
I am, however, pleased to report that the larger version of the C7 body (with the longer wheelbase) no longer clips through the chassis if the glued aluminum monocoque is selected.