If I recall correctly, It will take a bit more work to insert into a F or M body than a J body, because transmission crossmembers are different.
Before you start, I would find someway to get measurements first with car on ground or with car on wheel supported lift.
Measure one K-frame bolt to ground and the transmission slip yoke (the flat surface that slides into transmission) to ground.
Once engine is bolted in and transmission is held up with a jack (or something), re-measure both points. The distance of the pre-selected K-frame bolt might be different - due to weight difference on engine. If so, add (or subtract) that measurement distance to to distance of slip yoke to ground.
Raise (or lower) transmission on jack to know where to modify transmission crossmember at/to.
Note: Have the transmission crossmember made/modified a bit low, for you can always add shims/washers under mount to raise transmission up (easier to add than to subtract).
When done with your crossmember, you might want to take pictures and show here (with measurements) for I’m sure others will want to know what you did so they can make a duplicate.
I have a suspicion the NAG1 will be a somewhat common changeover.
I would - but would much rather go 5 or 6 speed stick.