Jrt

It would be easiest to purchase the mounts from http://www.engine-swaps.com/ because the stock 273 mounts are getting harder to find. The early A-body (pre-67) has the same k-member for the /6 and V8. The stock rearend would be a 7 1/4" and even thought the HP273 cars came with them I would change it for a mid to late 70's 8 1/4" rear end they are more reasonably priced than the 8 3/4's and will handle 300hp with no problem. The 8 1/4 will have the bigger 4.5" bolt pattern so if you want to keep the wheels the same for front and rear you can just use the same year vehicle as the 8 1/4 and use the disc brakes off of it.

The trans mount off of the 3 speed will bolt right on the 904 and bolt right in.

Welcome
Chuck