When I was much younger, I did a swap of a 454 into a 75 chevy truck that had a 350.
From your description, I would guess a timing issue rather than a fueling problem. I would take the timing to Top Dead Center and then see if it starts. You do this by using using a very large socket that fits the fly wheel and move the fly wheel (clockwise) until the timing marks to line up at zero. Then you can check where your distributor is lined up to make sure that it is going to spark off the first cylinder. You should also check to make sure the piston is at the top of it's stroke by removing the spark plug and seeing or feeling the position of the piston.
We ended up with timing at idle around 12 degree befor TDC at idle if I remember correctly (been so llllooonnnnggg ago).
We ended up going with an electrical fuel pump to add more clearance to the cross member compared to the mechanical fuel pump if I remember correcly. The mechanical pump as you found out sits mighty close to the cross member / frame and our 454 had a lot of torque and got uncomfortably close to hitting it when hitting the gas and letting off.
The HEI unit is a great suggestion too.
