Dave,
Epoxy primers are not considered a filling primer, its more of a sealer then anything. (isocyanate free)
Primer Surfacer is a filling primer ( and used for filling & blocking to make smooth panels)which have isocyanates,and most refer to these as K2 primers .

This is what I would do if it were me:
1: Strip down to bare steel.
2: Spray DP 40(graygreen), Dp48( White), Dp50 (Gray),DP74 (Red)or DP90 ( Balck) This is PPG's Epoxy primer.
3: do my bondo work over the DP sealer, and yes it is highly reccomended to do this by PPG's people!better adhesion of the bondo.
4: when all body work is done , Spray 4 coats of K-36 or NCP 271
Primer Surfacer.( another PPG product)
5: Block sand with 180-220( and these primers sand real nice)
6: Spray 4 more coats of K-36 or NCP 271 & block out with 400 grit.
7: Then panel should be ready for paint!!

I would stay away from Lacquer products as they are 25 years behind times & the Paint industry has Improved greatly since then.
These products dont have the shrinking And soaking up like lacquers do, plus they have a tad bit of flex to them.
remember the old days of lacquer and all the little cracks in the paint , this is the reason why, metal expands & contracts , lacquer doesnt expand & contract.

Do it right the first time .

SprayTech